There is no reason why a properly adjusted motor should tap. Now the book number is a general number. Not all valve stems are going to grow to that magical .005 mark. If you adjust a running motor so that it does not knock that means that you are simply dialing in the valve train for THAT specific motor and not going with a generic number that mechanics of varying quality can use.

Its ALWAYS safer for a dealer (even in the 50's) to point at the book and say 'I adjusted to the factory spec' and not 'I adjusted to YOUR MOTORs valve train' if there ever is a problem.