Domain Mapping And SEO, Worpress Multisite with subdomains

I have been searching other places about domain mapping and seo.
My question is if domain mapping will negatively affect seo rankings?

For example, I'm planning to create a blogging network with WP-MU installation using subdomains.

I will be creating pre-configure sites to sell my team so they can have their own blogs so then I can teach them on a members area how they can rank for diffrent keywords.

My #1 concerned is SEO and if domain mapping will not work well for raking their main domain?
or which url will ranking on googles' eyes the "www.subdomain.domainname.com" or their "www.domainname.com" ?

I'm planning to sell the sites with prosites pluging.

Thanks for your help in advance!