Changing up your usernames is an easy way to increase your security and privacy. Even if your public-facing accounts need to have the same username, using unique usernames for other services adds another hurdle for spear-phishers or other pokers-around.
By Matthew Malinowski, 2012-11-20
One of the best things you can do for your own security is to not reuse passwords. (The logic being, if you reuse passwords, all of your accounts are only as strong as the security of the laziest site you use the password on.)
You can increase your security and your privacy by not reusing your usernames either.
The security part is about the same as for passwords. If you use the same username everywhere and one site's username data gets compromised, the people who wind up with that data have one more piece of information about you. (It's not far-fetched to imagine someone talking their way into one of your accounts if they know your basic personal information and your username.)
The privacy part is a little different. If commonly use a username around the internet, try this exercise: search for it on Google. Even if you keep the search results for your name pretty well pruned, you might be surprised by what you find with a username search.
When you sign up for a new service, think to yourself: is this something I want publicly associated with a username that's reeeally easy to associate with me? It might not even be something embarrassing, it might just reveal a bit too much information about where you live or what your interests are for your comfort.
Choose a random username! There are all kinds of ways to choose random usernames - try password generators, or for actually readable names, try things like the suggestions in this SO answer (blue_butterfield) or of course, the Wu-Tang Name Generator (arrogant_ambassador).