Well, it is perfectly possible to have accurate facings (more accurate and consistent than hand work) by using machinery. However, the general scuttlebutt seems to be that few if any mouthpiece manufacturers that use machinery for final facing spend the time, money, and effort to dial in the process to achieve the accuracy and consistency that high grade machine tools could achieve. For something as small as a mouthpiece facing, in a soft material like brass or hard rubber, you could definitely hold tolerances in the range of 0.0002" if you wanted to, maybe tighter. But you need real engineers and machinists to get there, plus good high quality tight equipment.
The only way to answer your question for a specific mass produced mouthpiece is to do the measurements. If it meets the generally accepted tolerances then you don't need to have anyone work on it.
