Imagine you are installing the usual 4x4 inch metal boxes with mudrings behind drywall.
I was taught to install the boxes with a 1/8" setback. The boxes with integral mounting bracket actually have a little "spacer": the very end of the mounting bracket has two little pieces of metal sticking out; if you make those two flush with the edge of the stud, the box itself will be set back 1/8". Then use a 5/8", and after drywalling with the standard 1/2" drywall, the mud ring will be flush with the drywall.
Advantage of this technique is that otherwise, the mounting screws on the box will dig into the back of the drywall, making the drywall bulge out. Also, with the setback, you have a little bit of wiggle room. Otherwise, if the box itself sticks out from the stud even the smallest bit, you'll have a bulge in the drywall.
But there is something bother me about this. If this were really such a good idea, nobody should be using 1/2" mud rings. But if I look at the Depot or at Lowe's, clearly the 1/2" rings are the most commonly used ones. Why is that?
Another problem: What do you do if you have to put a box in a wall that has 5/8" drywall? Using the above idea, you would need 3/4" mudrings, which are hard or impossible to find. And what do you do if you have two layers of sheeting (either fire-rated wall with 1" of drywall, or 1/2" of drywall over 1/2" of plywood, for a shear wall)? For that you would need a 1 1/8" mud ring, which doesn't even exist (they make 1" and 1 1/4"). Right now, for 5/8" drywall I cut the spaces of the bracket and use 5/8" rings, and for drywall over siding I'm cutting an oversize hole into the plywood, and installing the boxes with the space cut off the bracket, and then use 1" rings.
I bet there is more than one way to do it right. But what do the professionals do? And what is the most common "right" answer?