My twins are ten now. One was born head first and the other was double breach (bum first) I never gave any thought to head shape, but now you've mentioned it, head first twin has a very round head and bum first has a slightly... More oval shape. The thing is, it doesn't look wrong or weird or anything... They're non identical, so look as different as chalk and cheese anyway, and honestly they are both gorgeous.
I do know a couple who's baby's head shape wasn't to their liking, and they had her fitted with a special helmet to mound her head. Not sure how it turned out.
I also know people who've had very placid babies who were left to lie in their cribs for long periods and they developed really flat backs/sides of their heads.
