I think I've regained the level I lost when I died every time when I got Raise. Don't worry about it. In the early levels when the whitemage in your party don't have raise, you lose just an insignificant amount of exp. Later, when you lose a significant amount of exp, whitemages always have some sort of raise, which reduces the exp you lose a lot. Be prepared for a bit of grinding though, you'll have to if you want to get though the cool missions.