I hope it's not too important, as I didn't really push it with either of my girls. Elyse REALLY hated it, and so I didn't bother. She's three now, and seems to be fine. Marieke rolled on to her tummy on her own earlier than Elyse, and didn't seem to mind it as much. She was about 6 months old by then, though. She's crawling all over now, and has no troubles that I can see. Neither of them have flat heads. I'd say don't get too stressed about it. Try it for a little while - your baby might learn to like the new perspective, but I don't really think it's worth getting stressed about. You can always ask your MD at your next appointment.
Have a great day!