Once your baby's neck is strong enough to lift her head on tummy position, it is normally safe to let them sleep on their tummies. This is also provided she is sleeping on a firm mattress because mattresses that're too soft usually makes it hard for babies to change their position.
Incidences of SIDS is more common with co-sleeping babies, ie babies who sleep on the same bed as their parents, than babies who sleep on their tummies.
I would say, let her be since she is already proficient in flipping both ways. My boy sleeps all over his cot when he learnt how to flip.