I do.
If you have to sign a copyright assignment statement, I'd think you'd be required to by the project (such as for GNU projects). But even if you don't assign your copyright to the project, I have a hard time understanding what using a pseudonym would do for you.
If your employer owns anything you create (like most businesses in the USA), then even if you submit under a pseudonym, once the employer finds out what name(s) you're using, they still own the copyright. You just make it harder and more annoying for them to find out that it's you.
I can understand if you're just writing a single bug report or sending a one-off email to a mailing list asking a question, sure, using a pseudonym is probably fine if you're worried about spam. But if you're actually contributing to the project (code, documentation, specs, marketing, etc), using your real name is the only way to go.
Also, if you put on your resume that you contribute to an open source project, I'd go check that out. If I can't find your name anywhere, I'm going to assume you're stretching the truth on your resume, probably in other places too.