I went through a ton of online nicknames before I finally settled on Lunarkitten, which is the only name I've had that isn't fandom-inspired. Fandom nicknames don't necessarily make you childish and some people tend to keep them for years, even after they lose interest in the fandom. I use Sailor Cybertron on most of the Sailor Moon sites I visit to represent both my awesome lifelong fandoms. Changing your name a lot IS pretty annoying to me though.

tl;dr: Screen names are srs business.