Industry standard blocking placement for shower mixer valve?
Different brands and models of shower valves require different wall-depth installation. You should always follow the installation instructions that came with the valve assembly. 

Most valves come with a temporary plastic or cardboard "plate" that illustrates the location of the finished wall surface in relation to the valve-body installation depth. If you do not have that "template" anymore, you can often estimate depth by assembling the trim to the valve and measuring from where the wall surface would be to the mounting holes. You need to locate the supply piping according to final installation depth of the valve, not the other way 'round (not an issue with flexible supply tube like PEX).

Except for special application products, they are made to fit a standard wood-frame wall but may not allow the use of 2 x 4 lumber as a mounting support; in fact, some recommend use of a manufacturer-specific mounting strap/stringer.

Example of depth mounting instructions for a random Delta valve model: