I sleep on my side most of the time, though I will occasionally wake up on my back.
Funny - after my recent move I got my own bed/room (tatami & shikibuton). I most definitely noticed my hips and thighs are not the same as they were a few years ago. The bed is pretty firm so I usually lay on my back until sheepy time then roll on my side to pass out.

Add: Oh, I didn't sleep on my tummy much after I hit my late teens.