We started defining and evaluating the process of shimming our lens mount. There are still a few unknowns though, like if photography lenses are actually manufactured accurate enough to benefit from shimming at all or if its something only expensive PL mount lenses need. If you have an opinion or ideas about this please get in touch.
Shimming if often used in PL lenses that have a scale for focusing. This scale is calibrated with a target like a Siemens Star or a Test Sheet.
Companies like P+S Technik do this with a rig where you mount the lenses and you can measure the focusing distance from the sensor to the target.
Once this scale is engraved cannot be changed as far as i now and that's why people shim the mounts.
Other cameras have "back focusing" that moves the sensor back and forth.
Photography lenses don't have this scales so they don't need shimming unless you want to change the range of focus to allow the lens focus more at macro or at infinity.
I think it's the correct solution to have the possibility to shim the mount, because otherwise you may need to move the sensor or try other more complicated ways to move and align the lens.
I found this image that show one test made by P+S technik, it seems that they project a pattern to check the focus of the lenses.