What if you write a great query letter, but you don't want to sign your real name to it because in the craftless days of your writing career you threw up a zillion sites and there are very bad samples of your writing floating around the web under that name. Is it OK, to use something else and only tell the agent after they read a partial?
There's no reason to do this. First of all, we only use a search engine if we're checking up on some weird or suspicious credential. Second, even if we did in theory actually find your writing and look at it, if it was totally different from your query, we would assume it was either another person or some old, unpolished work. Heck, I've got fanfic from when I was 15 still up on a website somewhere. If a older, bad version of your manuscript is online, do try and take it down, but you're probably worrying over nothing.
You should ALWAYS sign your real name in a query letter unless you have valid legal or political reasons not to do so.