Doing a complete remodel on upstairs bathroom. New tub is set, putting down Hardi backer on the floor. Do you cut the hole in the backer board around the sewer pipe to fit right up to it, or do you leave some slight space away from the pipe, but yet under the flange? I'm tiling over the backer board. I've read already that the flange should sit on top of your finished flooring. My questions is, do you screw through the flange holes and into the backer board? Or does the toilet merely sit over the flange (wax ring also) and just the 2 screws for the toilet go into the flange ring? Given that the flooring will be level, and the flange is already solid into the sewer pipe, is it necessary to screw through the holes in the flange itself to secure it to the subfloor?
You should screw the flange through the floor, through the subfloor, and ideally into some kind of framing. Use all holes. I don't fully understand the reason for this, but all flange manufacturers are very clear that you need to screw the flange down.