I'm probably preaching to the choir in reading what everyone else has said, but I was told by the DR and in reading articles that once they can roll over they can rouse themselves if they're not breathing.
I also agree with the comment about back sleeping not being safe. I also know someone who after having her BF leave her when she got pregnant came home from the hospital and the first night the baby was home it spit up and was on its back and aspirated. The mom woke up and found the baby was blue, started CPR and they decided a few days later to take the baby off life support.
Being on the back is safer, but not fool-proof, but, if your daughter is rolling she should be fine.
I have found however, that when my daughter got used to sleeping on her stomach when I put her down on her back she would cry for a good 20 mintues before settling down and if I went ahead and put her on her stomach she'd only cry for a couple of minutes before crashing out for the night.
Good luck!