For some undefined reasons, I always like to keep my domains seprate from my webhost.
May be I am bit paranoid, but I have heard that webhosts create a hell lot trouble if you want to host them somewhere else.
Domaisn are like valuable real estate. Keep them as much protected as possible. I always trust Godaddy and Namecheap when it comes to domains.
EDIT: I just realized that I never used the free domain registration feature which dreamhost provides. I am hosting my sites on dreamhost since last 2 years.