The studs loosen up more at an autocross then the bolts, but once they get set, they stay set. On the track, I'd check them between sessions (and if you get a chance, drive around on them after putting the wheels on and check them before the first high speed session). I'm running 710's, so that's about as much abuse as a tire can put on studs. As they have gotten older, they have moved less when I check them.
Every time I take the nuts off, a few of them do back out a bit. I always hand tighten them before putting the wheels back on. Remember, they are hand tightened, not torqued, as they have no bearing surface.
-Guy