Tummy time is so they get stronger at holding themselves up and it is also to prevent flat head. If a baby is constantly sitting in a seat or laying on their back they will get a flat spot on the back of their head where their head lays most of the time. Laying your baby on you, tummy to tummy is good enough to promote good tummy time. I think so is holding them upright up against your shoulder often. The shoulder thing is Rebecca's favorite position and her head is nice and round.