Update 10/30/2012: See comment I left under gregmac's answer
I just finished installing QuietRock 525 over existing walls. Half a sheet of 525 is heavy backer board and I am thinking that's the reason why several of my screws are having so much trouble recessing into the wall. If I had known in the beginning what I know now, I would have installed the QuietRock as the first layer and used special backer board screws with a square-shaped bit since this phillips bit wants to slip out of the screw head with a loud "ratta tat tat!".
So now I am stuck with some screws that stick out slightly from the surface of the drywall. It's not as much as 1/16" or anything, but you can feel it raise as you take your finger over the head. I could remove the offending screws, drill out a wider hole, and screw them back in, but I don't want to deteriorate the integrity of the grab.
I'd like to finish over them so that the wall is smooth. What do I do, tape over them? Use an extra-thick layer of mud? Eh?