Yes, using www works both for domains and sub-domains and you can choose which you’d like to use in your links and more.

For instance…

www. you.xtreemhost.com & you.xtreemhost.com will both load the same site you.xtreemhost.com

– or-

www. yourdomain.com & yourdomain.com will both load the same site yourdomain.com

However, we recommond both for simplicity and for good SEO that you only use either yourdomain.com or you.xtreemhost.com in your links and drop the www altogether which also results in increased speed for your site.