The back of my DD's head is flat, because I followed her doctor's advice (2 different doctors...) and didn't get her the helmet when I had the chance. I noticd the issue aroun 6 months, and was told to wait and see. We moved, and I brought it up to the next doctor who also said to wait. He kept telling me it would correct itself; which it never did, even after I became almost fanatical about not letting her sleep/lay flat on it.
Now she is 5... You can't really tell unless she wears her hair a certain way, and it is purely cosmetic... But I have a REALLY hard time finding helmets she can wear, no headbands will stay on her, glasses have to be super flexible or they push into her skull too much. I REALLY wish I had gone with my gut and gotten the helmet. Better to fix it now, when his skull is still growing fast enough to shape. To fix my DD's at this point would require surgery... Too traumatic for me to go for. (Not to mention expensive...)