I've gotten into the habit of rewarding DPS more often, unless the healer or tank are just that much better (not just in performance) than everyone else. Having played all 3 roles, and experienced the "glorious" random players that DF can often give us, you start to notice that finding a good DPS is actually pretty damn rare. Again, it's not so much in role performance (i.e. doing top damage), but also in how they handle themselves as part of the group. Team player mentality is rare.