It's simply not true that the guards are always set flush to the finished surface: it depends on the valve. It's always a bad idea to set the valve
at one extreme or the other of the allowed depth: there's always some leeway, and you should use it. You can calculate the wall finish depth closely
enough, but you gotta have at least some idea if you're using 3/16 mosaic tiles or 1 1/8 stone slabs, etc.