We started at around 2.5 months when she had completely mastered head control. She's 5 months now and still not rolling over :cry:
We started out by allowing her to sleep on her tummy during naps only. At night we would let her fall asleep on her tummy, then roll her back onto her back the rest of the night. At around 3 months she woke at 3 am cooing and playing on her back, my DH got up and rolled her onto her tummy, she fell back to sleep for the rest of the night. We started putting her to bed on her tummy and never looked back.
I think it's a decision you have to make yourself. I think a lot of babies do sleep better on their tummies for a few reasons regardless of the information out there that says that it's not true. However, I wouldn't let any baby sleep on their tummy unless they've at least mastered head control, and even not then if they were born premature, as the risk is higher.
I think the best position is side sleeping at that age, however. But that's my own opinion, and I'm not a doctor
*HUGS*
Sonya =P