My daughter has a flat spot on the back of her head,and it bulges out a little on each side. When she was younger, I asked the doctor about it... I was told that it wasn't severe enough to need correction, and it would fix itself. I really regret listening, because now she is 4.5 yo, and it is too late to fix without surgery. There is no medical reason to fix it, as it is purely cosmetic and doesn't have any effect on her development in any way.... Most of the time you can't even tell because her hair hides it. buuut, it is really annoying. She can't wear most pairs of glasses-only ones with extremely flexible frames, otherwise they dig into her scalp. (And chances are, she will need them when she is older.) headbands won't stay on her head, and certain hairdos look goofy as heck. The biggest issue is that I have a really hard time finding helmets that she can wear. They are not made to fit abnormal head shapes, so it's hard (and expensive...) to let her ride her bike, roller skate, or do anything else that would require a helmet.
The helmet to correct the head shape is normally used on kids two years and younger, because the head is growing fastest at that age. If you are going to look into it, do it now! Any later, and you will be in my boat.