It's like when Bob Dole was aked by MTV, "Boxers or Briefs?"
Answer: "Depends"
If I'm being ultra-lazy and using a premade crusticle such as Boboli, pita bread, english muffin, etc., then yes, I definitely pre-cook the sausage, mushrooms, and onions (the peppers I ALWAYS cook -- they're much better that way). This works especially well for pizza-on-the-grill during the summer, as the bottom gets toasty as the top melts, but you'd never cook pork sufficiently in that time.
If I'm starting with dough, raw sausage works fine, it cooks as the crust does.
What is patriotism, but the love of good things we ate in our childhood?
-- Lin Yutang