although you may sleep better knowing your baby is sleeping on her back once children roll from supine to prone (back to belly) there is no controlling sleep position through the night. although there is no harm and possibly some psychological benefit (for you) in flipping her back onto her back you should not feel like you need to do this. i assume your daughter can easily clear her airway in prone and i also assume she will very soon roll from belly to back.
my 4-month-old has started rolling from back to tummy. is that a worry when she sleeps?