I just just talking about this last night with a few friends. I think it is up to the guy. Some guys think that is a feminine thing to do, and some guys feel more comfortable with them shaved. To me, it makes no preference. I'd rather it not be out of control, but it's normal for guys to have hair under their arms, so I don't necessarily prefer shaved armpits.
What you want for yourself is what's most important, of course, but from a girl's perspective, totally shaved armpits on a guy freak me out a bit, to be honest. Trimmed is fine, as Timeus said, to get rid of the pit-fro, but completely shaved looks strange to me. Unless you're a swimmer or something, which is a different case entirely.
I think it would creep me out a little bit if I saw a guy with shaved armpits. There is a guy at my job that shaves his legs and he gets made fun of ALL the time. I think women just expect their men to be hairy, at least a little bit at least.
I'm 20, I shave and I do it for many reasons. I shave because it looks better, virtually no BO, I'm a swimmer and a beach go-er. I also do a lot of outdoor activities and it keeps me cool because, for me, heat build up under my arms. I also shave my chest and stomach too. I also have long hair so it was not the best looking either so I said out with it.