Some flaws are(seem) easy to fix, like missing hands.
do you keep strict balance, that characters must have equal number of flaws and merits in the game, even if they fix their flaws?
One view is to balance merits with new flaws, another is to reward good roleplaying with healing one minor flaw per bigger story.
I seem to recall that things like missing hands (and blindness, deafness, etc) bought as flaws are considered to be part of your essential nature, and so they cannot be easily fixed (although work-arounds might well work I guess), check out page 36, last paragraph in the left-hand column
Issues like that arising in play however can often be fixed fairly easily,
A better example might be something like "enemies", though a storyguide might be able to come up with a few side-effects of simply having them whacked 
Simple answer: no I don't, I just ensure that they have to work hard in order to mitigate the flaws, or gain the virtues (and apart from initiations I don't really bother "issuing" virtues and flaws in-game, the players know what the situation is without needing one).
I can't see that I have ever made a hard and fast rule like that, but I would certainly prefer that that is the absolute fastest that they can be resolved.
If you take the flaw missing hands (like I did for one of my characters), you take it in a way that cannot easily be fixed.
Reasons for this:
A balance of virtues and flaws makes a character round, and as long as you can participate in and contribute to a game, being the "strongest" is of little importance.
If a character resolves a story flaw after hours of playing (or the Mentor/True love flaw dies), or the character changes personality, an imbalance could happen - but who wants to play a character without a reason to go adventuring. Think of Frodo: he got on a ship and went to an Elven Land: booooring!
There are Mystery virtues, of course, acquired through initiating, but few are as "useful" as a good formulaic spell or two. More like a carrot and stick thingy to keep power gamers happy.
The answer to this is on page 36, in the general intro to Virtues & Flaws:
[i]"Flaws come in two broad types. General Flaws hinder the character, Personality and Story Flaws enhance stories. ...If a General Flaw will not hinder a character, it is not a Flaw, and the character should not be allowed to take it...
Some Flaws, such as Blind or Mute, could be easily fixed using Hermetic Magic. A character with easy access to such magic can only take such a Flaw if there is some reason why it cannot be fixed, such as that it is part of a character's Essential Nature (see P. 79). In general, characters cannot take flaws that will be quickly removed in play."[/i]
It's that simple.
So, if during CharGen a mage takes some physical Flaw and then whips up a quick fix for it, one that would make it difficult for the SG to imagine any reasonable situation where it would be a hindrance (or the SG is simply not interested in going through the narrative gymnastics to achieve that situation), then disallow it. If this occurs later, during game play, the best solution would be to rule that another Flaw must be taken to balance that lost flaw (similar to how defeated Enemies or a lost Animal Companion might be treated).
(And a wise SG would make this very clear from word one for new players, or players that they know tend to lean in that direction in other games.)
Removing Flaws is a houserule - you're welcome to go there, but there should be other rewards that your players would appreciate more (or simply balance those removals against undeniable new Flaws acquired during gameplay, such as new enemies or wounds, or anything else that fits the descriptioin).
While we're on the subject, it's also suggested in that same section that similar rules be applied to Story Flaws. If, for instance, a mage wants to take "Black Sheep" or "Close Family Ties", then the SG must see that as something that will add to the story, and the Player be agreeable that their mundane family will be a significant and meaningful factor to a mage's life - or it's not applicable.
Honestly I enjoy flaws more than virtues, having some special abilities is certainly nice but I find I develop my character more around their deficiencies than their gifts.
I remember a blind Criamon mage who's eyes wept blood as his sigil - he wouldn't try to "fix" his blindness for all the vis in the Order. 
No, when taken as flaws there is some reason why such isnt easy to fix.
Have tested allowing healing spells at double, triple or thereabouts the level that would be needed to cure the same problem if it comes up during play, to heal problems caused by Flaws, its acceptable but still not a great solution.
No. If someone manages to do something spectacular enough to get rid of a Flaw, then no balancing needed.
Though story flaws tends to remain or be replaced by another depending on what looks suitable and how they got resolved.
Nah, make it hard enough to get those new Virtues and let them have it.
Never! If someone works out a way to get rid of a Flaw, thats one thing, but i think its a lousy idea to remove flaws "just because".
If the player can work out something, that over time will remove a flaw....good for him. We don't tend to worry about flaws as much... We always come up with something to provide stories for that character. I have found over the years that Flaws are simply the things that flesh out a character, and provide they type of stories that the player would like to play through. If they eliminate them through play, then our group will find plenty of other things (Flaws) to keep the character busy... (As Any good SG would)
My question on this would be something like Phobia Flaws..... if you went to the time and trouble to expose yourself gradually to for example, spiders, (maybe use spells to bolster courage or something to start with) then how would it be bought off if the GM let you?
ie could a suitable amount of xp be used as long as you played out the scenarios or used downtime to face your fears...
also, for removal of curses.... without the swapping to an equivalent flaw?
Not a very exciting story neh?
I guess you might allow it to be bought off using some variant of the initiation rules to simulate this? I don't think it should be allowed through XP though (of course, developing a personality trait like "bravery" might help the character face his fear, which might be apt?),
Perhaps if they had to spend a season or two working on their phobia instead of research, enchanting, study, etc.
However I like the idea of having to develop a bravery personality trait - that is something that would make for a fun story and. If spiders were included I think you'd have a good foundation for removing the flaw as well.
We have loads of other stuff in our RP for it never to be boring, we tend to do lots of character growth things, it's just.....
I suppose it comes down to individual SG preference... what I see from here is that some people have house rules that if you face a scenario where the Flaw is eradicated through play by some means, you either have to (depending on house rules)
so back to the Curse Flaw which I think was what I was really wanting some understanding on... if for example, you managed to find some NPC (through RP, time, effort etc) who could and did remove it (for a "price")
I can see 2-3 ways....
(which is the current stance....)What I am really trying to say is, that my SG would charge xp as well as the effort gone to (not for a straight swap of equiv cost flaws course) find a cure for a curse (or whatever flaw, just Curse is a better illustration)
so, how much per point of flaw sounds "ok" if they are going to do this whatever is arranged in game to remove the Flaw? (as I disagree with the exponential 7+14+21+28 per each point that is done at the moment)
I have seen the web Ars Fantasia house rules which suggest a flat 10xp per point as well, so really it's just all your thoughts on what I have to put up with...
I don't accept the premise of the question, or at least don't recommend it as a wise starting point.
Flaws are not something that any Player should believe they can "buy off", ever*. The Character may wish they were not burdened by them, and it's possible that they might overcome some (Enemies, Blackmailed, even Blindness) with time, effort, and a lot of drama, but those should then be re-balanced by new flaws (maybe the orphaned, innocent young child of an ex-Enemy becomes a dependent, or you now have a phobia about becoming Blind again) - 10 pts of Flaws are always 10 pts of Flaws.
(*at least, by the rules, that's not how they're designed. House Rules can always go outside the lines - if it works for you, for your Troupe, then by definition it's "right" for your Saga. Otoh, does the Saga then become a story about overcoming personal flaws, or something else, perhaps even something magical or mystical?)
If the SG begins to allow 10 pts of Flaws to become less, then Players will, naturally (and inevitably?), angle their characters to be able to lose/overcome flaws more and more easily, and more and more of them. It's one of those slippery slopes that once started is hard to then rule against. Imo, better to simply never go there at all. ysmv.
I am of the same mind. I'd even say that flaws are flavors, removing them makes your character tasteless.
OTOH, through mysteries you can gain virtues without a balancing flaw. Under a good SM it takes years of work to join the cult and impossible to join a second, but anything is possible if the troupe agrees.
EDIT: And (Mystery Cult) Lore is a good xp sink. Maybe you'd need to sink 50xp into Profession: Shrink to clear your phobia, and then you can become a mystagogue of the Shrink Cult to cure phobias. :mrgreen:
That sounds like a very small cult.
I let characters "fix" such flaws.
If a character has a Missing Hand, then it would be OK for him to even begin play with that "fixed" with a magical device --- because there is the possibility of that device breaking, being stolen etc, in play.
Likewise, it would be OK for him to "fix" it during play, via a Creo Corpus ritual, because then we get to tell a Story about acquiring the vis for the ritual, studying sufficiently (or paying someone) to invent the ritual, etc. And we can still tell a Story about the original hand turning up in the possession of an enemy and being used as a powerful Arcane Connection to thwart some other plan of our character.
ArM is about stories. Characters trying to fix and overcome Flaws is a very natural story; it gives characters a great motivation for actually doing stuff. If all the players just sit down, and say --- "right these are the Flaws my character has, and he is going to go about solving these in this way" --- then the saga basically writes itself. The storyguide saying to the players "whatever you do you cannot overcome your character's flaws" just stifles the creativity and involvement of the players in creating the saga.
I would recommend ignoring the nonsense about Flaws being part of Essential Nature. The Flaw is just an agreement between the players that the character has this feature which will help the troupe to tell stories about the character. Taking the Flaw is the player saying "this is an interesting feature of my character". Stories about the character overcoming the Flaw are just as good and interesting, as Stories where the character is somehow screwed by the Flaw.
Now that's going way too far for me. An Enemy is an opponent who should recur through a long story arc and be nearly impossible to get rid of. If you do manage to get rid of the Enemy, that's a cause for congratulations. I can't imagine sticking the character with a new flaw at that point just out of some sense of karma. The same with Blindness. It shouldn't be curable with standard Hermetic Magic, because that would render the flaw pointless. If you manage to find some way to do a cure, again, good for you. You've told a good story in the process. You shouldn't be penalized for it.
The enemies apprentice/best friend/whatever...
There are usually ways to make the "new" flaw part of the story rather than just tacking it on,
Then why even give them Flaws to begin with? 10 pts just... because? How is that karma any less arbitrary?
If you agree that a character should have Flaws to balance Virtues, and understand why they have those flaws, then your position is kinda self-contradictory, neh?
If you don't, then just give your characters nothing but Virtues and be done with it.
(And I will decline to make the invited comment regarding your imagination.)
The Flaws are there to make interesting characters.
Characters overcoming their Flaws are interesting stories.
The process of characters getting rid of some Flaws is much more interesting than characters just having those Flaws. Characters have 10 Flaws to begin with so that there is some longevity and complexity to the character.