After killing my knees using 6 or 8 inches for some years, I moved to just doing step workouts on the floor--no platform at all. A few years ago I started using the platform again, and felt so accomplished for moving up. I will stick with just the platform for my step workouts--I feel the same way: good workout, no knee pain, no tripping hazard...
Today I actually put risers under it for Cathe's ICE Bootcamp (since there's no choreography). I stubbed my toe a couple of times
Funny how your body gets used to the various heights.