Finn naps on his belly but sleeps on his back for the night.
I was paranoid for a bit but then I read an article that made a lot of sense to me. It said that the reason it *seems* the back-to-sleep campaign decreased the sids deaths by 50% is actually because back in the day they didn't do autopsies on all infant deaths. A baby would die by say suffocation but they wouldn't do an autopsy, they'd just call it sids.
And I don't know if it's completely proven yet, but they're saying that babies that die from sids have a brain abnormality.
So I let my baby sleep on his belly for naps and I check on him a million times. He moves around a lot and turns his head from side to side and there's nothing in the crib with him, so i'm not too concerned. At night he's on his back, he's thrown up (not spit up, but actually vomited) while sleeping that way and he's never choked on it. This is what works for us.
In the end, it's up to you and what you feel comfortable with.