Now as long as you have Fortune points you can add a Fortune Die to most checks and Fortune die rolls are closed rolls. This is fine and quite clear.
What I would like to known is when a Fortune die has to be rolled ?
Is it rolled with the + and - dice and the total calculated ?
Or can it be rolled and added in after the swerve has been calculated to add to the swerve (if it is needed) ?
So what I am really asking is does the Fortune die get committed before the action check or can it be added post action check (only if needed) ??
Well, if you are playing first edition, before.
While I have the PDF of second edition, I have not familiarized myself enough with the new ruleset to say
I don't see anything in the Feng Shui 2 rulebook which specifies, but we play that you can choose after you've rolled your Swerve whether or not to spend a Fortune point on the result.
So if you roll your positive and negative dice and get a Swerve of -3, you can decide to add a Fortune die to get you over the top into a positive Swerve.
On page 11 you can read the following text :
...Except where otherwise noted, you spend 1 Fortune
point to add a Fortune die to any check. You roll an
additional plus die and add it to your Swerve.
on Page 10 you have the definition of a Swerve :
In any Task Check, you roll your dice and then
subtract the minus die from the plus die. This
number is your Swerve
It seems to indicate that you decide AFTER knowing your swerve to add or not a Fortune Dice.
But...
On page 104 in the dodge with fortune die example, it's clear that you decide to add a fortune dice BEFORE knowing the Swerve result of the agressor.
You pay the shot cost and also spend a Fortune point,
reducing your total Fortune to 2. You roll a 3,
increasing your Defense further, to 18. The GM
rolls a Swerve of 2 for Rance, which adds to his
Martial Arts of 15 for a result of 17.
Even if it's in an interrupt action, game balance suggest that you should make your choice always before knowing the final result.
Well that's my point of view 
So according to this interpretation, on an attack you can roll swerve, decide to spend a fortune point, then find out if it succeeded?
The rules seems to says that but that's not how I want to use it, I rather like the decision tu use fortune die made before the Swerve result is know.
Fortune die should not being used only to escape bad fate (counter a bad Swerve, be afraid of a Boxcars result,...), but to try to do awesome things at the rigth moment.
Seems to me more heroic.
Maybe some clafirications in this rules will come later.
PS : sorry for my english, it's not my native tongue.
When reacting to a potential attack, roll the Fortune die to add to your dodge.
Nothing wrong with that, but I have hardly seen it ever happen in my games.
It would only be used in in extreme circumstances where your character is nearly out of it anyway, if you have next to no injuries why bother ?
So is it important when you roll the fortune die for dodge ? Irrelevant really I think.
When attacking I am all in favor of rolling a Fortune die after serve has been calculated. It is really not that important and gives a player that little bit of an edge.
There is nothing worst that spending a fortune die then rolling a 6 on the negative die and the hopefully "awesome" roll turn completely pear shaped 
Player soon guess the target numbers required to take down mooks and GMCs in a combat, allowing them to roll a fortune die then they are missing by a whisker, or need a positive serve to power a schick, only makes the combat move faster, always a good thing, and makes the player/character more awesome.
I agree. This also moves the game along faster in terms of skill checks to do plot-activities (finding stuff out, breaking into the lair, whatever). Thirdly, this more powerful interpretation of fortune is IMO on par with the power you get out of schticks which also cost a fortune die. With the weaker version it's a no-brainer; in situations where both the schtick and a fortune die could give you a benefit, you are pretty much always going to spend the fortune on the schtick. But with the stronger version, maybe not. Maybe, in fact, you activate both. 