The International Cessna^® 170 Association

Nylon Locking Nuts are intented to be used once.
Take into account that MS nuts are qualified by proving that they can be used a minimum of 5 times.
Knowing this, I have no problem taking one off once to add or subtract a washer.

The next issue is ones that have been exposed to the elements and UV. If it has been a while, I replace them.
Jim

A nut that's properly torqued against stationary fittings will not normally loosen unless vibration is present. The elastic band within the crimped end of the "nylok" nut is designed to counter any vibration-induced tendency to loosen. Only token friction is required to cancel vibration-induced unthreading and that, I suppose, is the justification for the re-use specification.
The nuts are relatively cheap so when I remove them I usually throw them into an old coffee can and re-use them on household/automotive projects, and re-install only new ones on the airplane.