Three people already knocked it out of the park, but I'll put in my two pesos.

More or less, whenever I am GMing (read; oh my god I need a break), I have a player character on the go that I treat like any other player's character while also doing the GM thing and moving the story along and creating/ playing NPCs for everyone's benefit. I basically started GMing because I never saw games I wanted to join and I was tired of the ones I did join dying before they got started, which is my least favorite thing about roleplaying. I'd hate not having at least some player writing on my end because I approach roleplaying as collaborative story telling than something like d&d where the GM is just doing the narration and encounters.

I've always just used my single account for it all. It's actually the norm for GMs to have a player character, and sure it kinda sucks that you're never going to get surprised by a story you're plotting out, I'd argue it's more rewarding to actually come up with something that your players react strongly to. I personally don't use alt accounts because A) I'd rather just stay logged in rather than logging in and out to use different accounts, and B) There's always a slim chance that I'd end up taking a name that some new user wanted for an account that seldom gets used. I'm not saying people shouldn't do it, I'd just personally feel kind of bad if I ran across someone who had a crappy variant of the name of the alt account that I was mothballing. Your user name is your long term identity and all that, you kind of try to pick something you're happy with.