and in any case there is no correct way to do it, as piatkow has mentioned it is convention that example.com resolves to the subdomain www.example.com however this is not a rule and it is not always the case.
Well i dont see a point of using www. prefix on Your domains, its not important at all
It does have it's uses. Some cPanels used to use example.com as the mail domain and then use "www" for the website. One of the largest ISPs in Australia used to do this, managed from a home-grown cPanel.
Users couldn't access the site using example.com, but they could use www.example.com.