IMHO whether it be plastic / metal / ABS or whatever.. the enclosure should not distort anyway.... or it will end up non square so the face plate will not sit flush?
I had assumed Theory was talking about when putting a plastic box onto a plastered wall, when you cant see where the bricks are...
and even with good drilling, sometimes when you drill the hole, it is smack in line with the edge of the brick & mortar,
So the drill can just drift a couple of mm up/down/left or right! and because of the moulded countersunk recess of the patress, when you tighten the screw up it can rotate the whole box around off square.
With a round head screw which is narrower diameter than the patress hole, you can tighten the box without it twisting.
Countersunk screws with cup-washers can achieve the same thing!
that was my understanding... maybe wrong? dunno? :|