How is www special?

I am running MU in subdomain mode, and I have it installed into its own directory. I am going to be setting up 20 – 30 sites, and it is not for public registration.

My base network URL http://examplenetwork.org will not be used. But I am planning to create a child site, http://www.examplenetwork.org to be used just as any other child site. Then I plan to use MU Domain mapping to point http://www.example.org to this site and this will be the public address for the site.

What special treatment does www get?
Will using http://www.examplenetwork.org cause me problems down the road?
Would it be wiser to use the Domain mapping plugin to make the site be main.examplenetwork.org and use MU to point http://www.example.org to it?

Thanks