Periodic injector removal for cleaning is an example, par exellance, of the Waddington Effect. If you can't or won't see it, you are part of the problem.
I have seen more problems from periodic replacement of oil filters than from fuel nozzles, but that is for another thread.
Although I believe you are correct in this case, as you are suggesting, the contamination is on the outside, covering the airshroud on NA injected engines. not a problem for turbo charged engine injectors. The workshops I am experienced with don't service the nozzles unless there is a problem relating to the nozzles.