If she has a great rack there are few things that will look as good on her as a tank top. So I'm going to disagree with Spain here. On the other hand, they let women run around with no tops at all on their beaches, so I guess that's better than a tank top.
More than anything else in the world, guys like girls. Some guys like tank tops, and maybe some small few don't even, but most don't give a rats behind. So, if you like tank tops just wear em and get on with it, because if guys aren't attracted to you, it ain't got nothing to do with what kinda shirt you're wearin!
If we're just hanging out around the house/apartment or as pajamas they're cool. Or if we're outside playing sports or heading to a pool. But to be honest, yeah they look a little trashy if they're worn at a restaurant or somewhere else that isn't super casual. Though I also think that's true of guys wearing tank tops. When I'm in Vegas and I see all the flabby middle aged dads wearing their NASCAR tank tops I shudder and hope I'll never think that's a good idea.
I'm sure some guys like them :) though not many gals wear them (well, I haven't seen many gals wear them) they may just not be very accustomed to women wearing them :D though they are just shirts at the end of the day.
What are you defining a tank top as.. something short, low cut, with straps not sleeves? (I'm sure there is different types). I own quite a few that I definitely don't consider to be "low class", I dress fairly classy anyways. They're so easy to throw on especially in the summer when its hot.