I prefer whatever is a girls natural hair color. If she's going to do anything, I don't mind a few highlights. I'm not a fan of the full-on bleach, because after a couple of weeks, all the roots start growing back and the color is uneven and looks odd. Natural or minimal highlighting is where its at.
I don't have a hair preference for girls at all as long as it at least LOOKS natural. The only time it's ever a turnoff for me is when a girl's hair color is a very obvious dye job or is styled in a way that looks like it was done professionally. I don't like the high maintenance look.