Please read: Matt Cutts: "Adding a 301 redirect can be an especially good idea if your site changes"
If I'm johnny my URL might be 134564/johnny and if I change my username to paul, and people have outside links to my profile at 134564/johnny, they'll still work. This means more than one URL to a page. Issue a 301 if the name doesn't match (doesn't waste more DB, as you need the username in the profile anyway).