Reasonable amounts of hair are necessary. I mean, a) it'd just be WEIRD to be completely hairless down there, and b) if you don't keep that shaved compulsively, it'll be scratchy for you and the girl.
The only hair-related complaints I've had are with shedding. It's natural, yes. But if I'm going down on a guy, I hate it when I get a piece of hair stuck in my mouth. And it kind of ruins the moment if I stop to try to fish it out. If my hair is down, I have enough trouble keeping it out of my mouth while I'm occupied.
Well unlike some of the other girls, I shave completely and don't feel 10. However, for my man. I like it trimmed or shaved. It makes giving head much more pleasant. Especially if you like having your testicles licked or sucked on. Nobody wants a mouth full of hair. So if you expect her to do that, you should trim or shave, definitely. It's different with each girl, though, so I think you should keep it neat and not like a forest and then ask the girl you are with.
See the thing is. If its long, it will catch everything in it. If its completely shaved. Well you'll look like your 10 again. And if if its short like a half and inch to an inch its better. Plus it will make you "manhood" not look so much like hippy.