At the homeclub of Strictly Sticks Traditional Archers in Alverton, PA, a DART system is set up. One of the shooting positions we use is sitting down, and it is often "looked forward to" as the shooting position where everyone does well.
We all practice at the bales mostly by standing, but, we do bring out the chairs for some sitting. If you have achieved a good, workable shooting form whilst standing, we've found that it easily translates to the sitting position.
I shoot a 69" Vigilante, and have NO PROBLEMS with whacking the floor. I'm 5'10", and believe that my adequate shooting style allows for sitting with no ill shooting effects.
Try it: if it works for you, you now have opened countless shooting possibilities and opportunities. But, ENJOY IT, FIRST!!