There is absolutely nothing wrong with doing the workout at 6 inches. In fact it's more common for people to step on 6 inches than it is on 8 inches.
You should step at 6 inches if you are cardiovascularly challenged on a 6 inch platform. You should also be stepping on a 6 inch platform if doing the choreography on 8 inches feels awkward (like you are going to trip) or uncomfortable to any of your joints (ie: knees). We all stand at different heights and are all at different fitness levels. So you have to find the right height that challenges you and is comfortable for your joints. For me personally 6 inches is plenty high for a cardiovascular challenge when I am doing trickier choreography. The fancy footwork, turns and travels keep my heart rate plenty high at 6 inches plus I feel safer doing the choreography at that height.
I like 8 inches when the choreography is more basic in nature or "blast" oriented with simple & repetitive explosive movements. It spikes my heart rate more quickly to a higher level for an interval training effect.
That all being said sometimes I have days where I just feel really strong and light on my feet and I enjoy doing a dancier step workout on 8 inches for a greater challenge and sometimes I have days where I am feeling more fatigued and I enjoy doing a HIIT workout on a 6 inch platform. So you really just need to be intune with your body and do what it can safely do each and every day you workout.