This is a topic everyone has been arguing about lately. There are advantages to both.

Reasons to use WWW

• Non-tech people need something to tell them “this is a web address.” The two things that tell them that are www and .com. If you don’t put the www in, then people rely on there being a “.com”. In an age when there is a shortage of .com domains, do you really want to reinforce the association between URLs and “.com”? You could use “http://” but do you really want to put that horrible-looking thing on your business card (or whatever you’re putting your domain on)?
• It adds balance to the URL. The three characters (plus a dot) even things out, as you have “.com” (or some other TLD) after the domain.
• When I want to go to Webmaster-Source.com, I type “webmaster-source” into the URL field and press CTRL+Enter. The browser adds the “www.” and “.com” into the URL. If you don’t use the WWW in your URL, then I have to wait an extra 3 seconds for the redirect to the www-less URL.

Reasons Not to Use WWW

• It’s redundant. You already have the protocol (http). Do you really need the pointless “www” as well?
• Your site is located at a subdomain. Subdomains look real stupid with www in fromt of them.
• You just don’t like the sound of “www.”

Whichever you choose, make sure you redirect one to the other. This is easily done with mod_rewrite.