It depends.

Johanna Rothman wrote a good article about the ways in which "it depends."

If you can figure out what kind of testing is most important in your situation, then you are one up on the game.

Otherwise, get one of everyone, to cover the all the bases.  9:09:35 PM    

It should be a no-brainer that the QA folks (especially the testers) and the Customer Service folks should work together (also known as ganging up on development).

QA has development's attention much more than CS, and CS knows more about what crazy things the customers have done or will do.

Kaner has some decent thoughts on how to justify this.  6:29:57 PM    

It's spring election day here in California. So I walked my son to school, and stopped at the polls on the way back.  8:55:15 AM