My ds started rolling over around 4 months or so, although I kept putting him to sleep on his back. He slept on a sleep positioner. Now he's 6 months, and I still put him to sleep on his back, but he pretty much rolls over to his tummy after I lay him down. And he completely rolls off his sleep positioner. So, I'm weaning him off of that completely. I've taken off one of the "sides", and am about to take off the other. He doesn't stay on it anyway, but I was using it for the incline more than anything at this point. I would like to get rid of it.
As long as your lo can roll over each way, I wouldn't worry about them sleeping on their tummy. We had about a week where ds would get very mad when he couldn't flip back over to his back. Then he figured out he could sleep on his tummy, and that's his preferred method now.
Oh, we also swaddled, but with his flipping, he completely came out of it every night. So, no more nighttime swaddling. And I'm weaning him off the daytime swaddle too.
HTH!