I have always hated using benches when I play the piano. In my tiny dorm room I play my digital piano on my bed, with a pillow to support my back. It's so much better than the bench I would use while playing my Grand Piano back in my home.
Since I play on my bed I can't reach a sustain pedal so I do this trick to get the sustain pedal always stuck on, I hold it down while the piano is being turned on so the piano thinks that on = off and off = on, so if I DONT want sustain, I just click it with my left hand :P
My back is pretty crappy. I have scoliosis. Anyone else have a poor back? Do you use a chair or some other support instead of the bench? Is the pillow somehow causing more damage than a bench? I have no experience/knowledge on this issue.