This topic is a very old an ancient topic, but I’ve arrive definitively at whether or not the main domain should have www, or not. The answer is “it should”.
The reason being, is that a cookie set at the domain level, exists for all subdomains. If you have subdomains, or ever plan to have subdomains in the future, it’s best to use “www” subdomain for your main site. It’ll pay off by saving you some headaches down the line when you have specialized subdomains in the future (blog, beta, members, etc.)