My second was (still is) a tummy sleeper. I think the general idea is that if bub can move into trouble they can usually move out. Even so, try and minimise things in the cot that bub could get tangled in. Can bub roll back onto his back? Can he lift himself up on his hands, or push himself about? I think those are good signs that he can move if he gets stuck. Is he a really deep sleeper?
Also make sure there's good ventilation in the room - one of the problems with tummy sleeping is that bub can overheat. But a slightly open window will help and I guess central heating won't be an issue in Summer. But in Winter, I'd have the central heating turned down really low in their room, just enough to take any chill off the air. One of my MCHN said something about SIDS incidences rising as more families used central heating - no idea how true that is though.