when should you have a fuel injector fuel flush done

This can be a touchy subject, depending on who you ask. Some believe these are necessary to maintain the proper operation of the vehicle and will help to clean the injectors. Others see it as a revenue source for shops that push the products of the oil additive/flush companies that provides little benefit for the cost. If you are having idling issues, rough running or hard starting problems, and you use low quality fuel, then I would recommend it. If your vehicle runs well without any problems and you use a good quality fuel, I don't see a reason to have the job performed.