As a publisher you will always here in articles its good for SEO if you make your site either http://youtube.com or http://www.youtube.com Well youtube doesn't have this! They still allow both urls. They probably don't care becuase there so big anyways Still I cannot log in using firefox auto login if I use www, becuase firefox only recognizing the pass on the http://youtube.com prefix Very annoying, I wish they would fix this. Tired of having to switch the prefix all the time to log in.
If your website is so popular and gets thousands of direct visitors each hour then it doesn't matter if you use these methods