Every girl have their own preferences, but here are some insight from what girls have told me:
"Men should have hair on them. Thats what makes them look strong."
"Its like a haircut, you want to keep it short, keep it neat!"
"Why did you trim your hair?"
"It depends how young / old you look.. If you have a baby face, I would probably think you haven't hit puberty yet"
I think generally speaking, having and keeping hair as a man shows your masculine side, and women are wired to be naturally attracted to hair as it symbolises Strength.
On the other hand, you don't want to look like an ape who hasn't evolved yet.
So really, its all about keeping it tidy and neat.
I trim mine, but not to the point where I'm hairless. I think of it as having a hair cut, and its all about the style