I always use a PVC mud ring since they have square corners.Especially most rings under 1 inch of depth that typically have rounded corners. Square cornered rings I can see being much more like using rotary cutter around a straight edged box and easier to get a close fit cut, I hung my share of drywall before, but mostly around NM boxes, those mud rings I can see being a bit more of a pain to cut out.
The other alternative is to just have the drywallers do correctly what their getting paid to.