I tend to run Ars Magica so that non-grog characters only die when their players want them to. That doesn't mean that I think that should be written into the game's rules. (Although it was, in an early draft, which is why the "dead" wound level does not appear with complete consistency in character descriptions.)
Similarly, if you think that botched Aegises will break your saga, house rule them to not botch in whatever way seems appropriate, or always choose botch consequences that do not break your saga.
Incidentally, my reading of RAW is that Mastered Ritual spells have no botch dice when cast under calm conditions, because the description under Ritual spells says "always use a stress die", not "always count as stressful". I suspect that that was my intention at the time, because a number of ArM4 to ArM5 changes in that section were designed to create a background where magi could use magic to do casual, everyday stuff without statistically exploding with unnerving frequency, but it was over ten years ago and I don't remember exactly what I was thinking at that point.