[disclaimer]This is just my opinion, based on personal aesthetic taste[/ disclaimer]
I think volume or height on top of the head (including the crown area), along with volume on the sides (but not to exceed the length volume of the crown by much) actually fits round faces really well.
Also, a hairstyle that isn't too "round" looking. Here's a look that I imagine would NOT easily work w/a round face (Eisenberg, below, has an oval shaped face though):
look isn't hot either w/a round face IMO. ... Although, to be fair, this woman has some other pics floating around out there showing her w/hairstyles that do suit her round/squarish face much better.
But of course there are so many exceptions, always. You may find that a certain look "shouldn't" look right on you, but ends up looking just great. Play around with some ideas. Straightening it may just leave you with a flatter look that you feel only emphasizes the roundness.