It matters depending on your audience and your off line marketing efforts.
The best is to use http://site.com because it avoids many potential problems. For example with 301 redirects. Also technically it makes sense, why the hell host your site on a subdomain, it should be in the root???
But we do use www. because our audience is an old and traditional audience, so they think a site with out www. is not even a site. So to maintain UX patterns we use www. But in future we will switch to the more sensical root domain.
