both of mine usually slept on their side & I'd try to swap to the opposite side for the next sleep to avoid the flat spot thingy that can happen.

They are both tummy sleepers now. I put DS in bed on his back but as soon as I leave the room he rolls straight over onto his belly so I just let him sleep how he wants to sleep now. We have the angelcare sound & movement monitor so that makes me feel a little more relaxed but I know that wont stop SIDS so I do check him & make sure his head is nicely to the side so he can breath clearly.