I think it varies. Some people put their real names, some their fan names, and some both.

My experience (which I'm sure isn't true for everyone) is that most people put their fan names, and were calling each other by their fan names even in real life (which is awesome).

And putting your fan name is definitely helpful in terms of recognizing people. There's usually lots of badge-peering as people check to see if they know each other online (and lots of squeeing when they do). Obviously that wouldn't work if people didn't put their fan names on their badges.

So, yeah, my advice is to put your fan name if you're comfortable doing so; you definitely won't be the only one!