Putting baby down on tummy?

So now that my 8mth old can roll and flip, sometimes she ends up on her tummy to sleep. When she does that, it’s the only time she sleeps thru the night. However, every night I still put her to sleep on her back, and the majority of the time she stays on her back. My question is, is she big enough for me to start putting her down on her tummy, in hopes of her getting a better sleep at night? Or do I keep putting her down on her back, and just have to hope she flips?