Hi Arch,
I can't comment on the use of drywall screws for faceplates because I
don't use them. Guess I took the original admonition to heart. Now, for a
couple of comments. I have found that a screw is likely to twist off when
inserting it, or, when removing it rather than from any twisting torque on
the faceplate. Some woods, such as oak and hickory, soon make you go to a
more sturdy screw. Also makes you get one of the little devices that drills
around the broken screw shank so you can get it outta there.
Now, I am going to add to your list of thought provokers:
Is there a benefit (real or imagined) to using a flat head screw, with the
taper underneath, that will center the screw into the faceplate hole?
Or, are flat shouldered screws better because they add a 'clamping' force to
the faceplate?
Personally I use the latter type (square drive, washer head #8 size by the
length that seems appropriate for the piece of wood about to be turned). Oh,
and once upon a time I predrilled all the screw holes but now just drive
them in.
And one last question, should you put a screw in "every" hole in the face
plate? Even if there are 6 or more? Why?

Jim in Ohio