I insert blocking inside wall cavities for all accessories. In the dry areas, I back them with plywood blocks behind plaster or sheetrock. In wet areas, like the hand shower, I back them with waterproof materials: brass, aluminum, heavy plastics... This is more of a design issue than a mechanical one. I ask my clients to decide in advance, how much wall space is needed for towel bars, shelves, mirrors, and other accessories. It is hard to pin them down when the room is gutted, as to where accessories will be mounted. But, deciding after the walls are finished, after the chance to insert solid blocking behind the substrate has passed, leaves no other choice than to rely on plastic anchors.