I tried to play with the guitar on my left leg, it was more comforable for my picking hand but I had back pain too because my back kind of twist. Anyway, I always play on my right leg, it feels more comfortable to me. Like Dan said, being slouched cause back pain too, in my case I had pain in the sternum region too because I sloched over the guitar for long period of time. Bottom line, I would say, too, to keep your back straight.