During the summer i keep my hair short enough that it doesn't matter. But when I shaved my head the real short hair would grab everything in my helmet and it wasn't that comfortable. It'd have to be about 1/4" or freshly shaved is the way to go.
Boywonder~ i don't think you have any of those options
I have a hair cut that looks better messy than kept, so the more I wear my helmet, the better my hair looks. You could always carry a spray bottle and a comb. I used to do that. Besides, you ride a bike, if you don't look at least a little disheveled and "uncaring" about your looks, people will wonder if you took the wrong bike today, and that maybe, just maybe, you should be on a BMW, or maybe a nice expensive cage, like a Mercedes Benz or something, ya know what I'm saying? Bikers are bikers because they crave a little more out of life than the fellow with every hair in it's rightful place, and matching socks and stuff. Let it slide, or make it work for ya....
My hair always looks like a girl Lyle Lovett so no one notices... I find when my hair is real short it pokes my scalp making my head itch under the helmet. Seems like a little lenghth is better. But I keep it real short for style and ease... dahlllings...!