I usually use 1 riser (6") for Cathe's workouts and 2 risers for Low Max and several other instructors' workouts. I used to use 2 risers all the time, but decided I'd rather add power than height.
For the high step, I use anywhere from 3-5 risers, depending on how much weight I'm using (more weight, fewer risers, less weight, more risers) or if I want variety .HSTA has SOOO may leg presses, to avoid doing the same exact movement over and over again, I try to change them up with almost every set, doing some at 14", some at 12" and some at 10", some off the sides of the step as well to hit the inner thigh more.