My 7 month old does the same thing. I can't put her down on her back anymore, it's either side lying after nursing, or rolling onto her tummy, and I've had to just realize that whether she's sleeping with me or sleeping on her own, she prefers it.
When she was sleeping in her crib a few months ago and rolling to her belly, I asked the pediatrician about it and she said not to worry too much. "You can't stay up all night watching her to make sure she doesn't do it," she said, "but if you're awake and notice it, maybe try to shift her to her side or back if possible..."
So I try that, and it usually means she wakes up...but I remind myself better safe than sorry...
I completely understand how you feel though! I think I'll always be worried about it!