Rolling onto stomach to sleep?

Lauren

I always put my daughter to sleep on her back but this morning when I went to pick her up she was on her tummy just babbling and lifting her head. She hasn't rolled from belly to back yet (that I have seen), so I'm wondering if I should let her sleep on her stomach if she gets there on her own, or try to check on her and turn her if I find her that way? At least until she masters the belly to back roll?