I think "damage their reputation" might have been a little strong as I was in a hurry posting that.
It's not that they would be marked for life or disciplined. It's just that I can't think of a good reason to ask. People know that I would not withhold such information for any reason once I knew for sure. So I would probably figure the person is greedy or lacks patience, neither of which would reflect favorably. Or, they may be the type of person who likes to have one up on everyone else - these are the ones who, after everyone else is notified, will say, "Oh, I found out a month ago about my raise." Who looks bad in that situation? Me. It's not that I don't sympathize with wanting to know - I'd want to know myself!
In my experience (and certainly everyone else may see it differently) the type of person who would ask that is the same type of person who eavesdrops on conversations, looks through other people's stuff, etc. [Nocturnal, I'm not saying your gf is like that; I'm only talking about my experience.] Others perhaps wouldn't give it a second thought if someone asked.
It has nothing to do with open communication, which I'm a big fan of. But when it's time, I'll tell them. Just to clarify a bit more, I would have no problem if someone came to me and said they were considering a major purchase (house/car/etc.) and wanted to know if I knew of any reason that it would be a bad move at the present time. I might be able to indicate that work is a little slow and they might want to hold off, or I might be able to say that things look really good for the next year. If someone asked me if I thought their work would justify a raise next year, I might be able to say I think so. I'm not pinned down and I would give an honest opinion. But asking what specifically their raise will be is not something I'm comfortable with.