Don't go by what Cathe, or anyone else, uses, go by what works best for you. The tallest you should use is to have the box set up so that the leg on top is at a 90-degree angle. But shorter is effective as well, especially if you squat down into the move a bit at the bottom, to pre-stretch the muscle.
I use anywhere from 10" (with heavier weights) to 14" and like to vary the height just for the sake of not doing the exact same move in the exact same ROM all the time. In HSTA, there are many, many, MANY sets of step-ups, so I vary a lot: some heavier on 10", some on 12", some on 14" with no added weight or light weight, some off the sides (more of a straddle).