Does your baby turn his head both ways? If so, chances are he won't get a significant flat spot. The problem typically happens as a result of torticollis, which is when the baby has muscles issues in the shoulders and neck and it's painful for them to turn their heads one way or the other. With my third baby, he would only turn his head to his right and even after physical therapy and daily stretching exercises he wasn't making enough progress so he ended up with a helmet.
So, keep an eye on which side your baby has a preference for, if he has a preference at all, and then just turn his head the other way half the time. Your pedi also checks for torticollis so they would say something if they thought there was a problem.