No matter what Cathe is using, it's safest to use a height that doesn't make the leg on the step bend at more than 90 degrees.
I vary the height of the high step from 10" to 14" for step ups (3-5 risers). For example, when I do HSTA, I vary the step-ups so that I do some with heavier weights and a lower step height, some with light or no weights at the highest, some off the back, some off the sides. I think it's always a good idea to not do the exact same movement over and over, to avoid overuse, and also to hit the muscles in a variety of ways.
I also adjust the number of risers according to the exercise. For example, when I use the High Step for sit and stands, I add risers to make it about 18" high (which makes more sense for me, as Cathe is 5'2" and I'm close to 5'6").