Electrical before drywall?
A rotary tool works great on plastic boxes. You just have to go the right direction around the box. I've never had a problem. You need to go counter clockwise with a rotozip, this way it follows the box. Don't set it too deep or it will cut through. It's much faster and more professional to use a rotozip. Just set the bit to the right depth, set a couple of screws into the sheetrock away from the box to hold it in place and drop through into the center of the box using previous measurements. Then move to the outside, pull out, and drop back in on the other side and move counterclockwise around the box.
Some people still use the chaulk, lipstick, lead pencil, etc which IMPOV leaves me scratching my head...but if you only had a couple outlets or didn't have a rotozip would make sense.