I’ll suggest you stick with one format for consistency. The reason is that say you use http://mydomain.com
When you mention your site in third party websites even for Backlinks, Promotion and while leaving comments on other blogs, google treats both the URLs http://mydomain.com and http://www.mydomain.com differently.
Technically speaking, although they point to the same root Search Engine does not know of this fact and therefore may penalize your site for duplicate content as it’ll feel there are two different sites with same content.
For SEO’s perspective its advisable to use one and stick with it. Also, use redirection from http://www to http:// or vice versa from the Domain Control Panel so that if by chance you use a different from somewhere the Page Rank benefits get transferred to the one you want.
As others have mentioned, consistency is key, once you make your decision stick with it. I personally do not use the www and it seems that most people are not using it as much. My advice is to drop the www