A tolerance of 10% (1/4 stop) in hole size is more than adequate. Don't forget that the 'optimum' hole size depends on the light wavelength, and as that varies over a close to 2:1 range so does the optimum hole. If one were to be ultimately anally retentive one would need one size hole for red subjects and another size for blue subjects.
I use my enlarger to inspect pinholes for roundness and size. Put the pinhole where the negative should go, put in a short lens and crank the head up. To make sure they have a razor edge to them I look for light reflecting off the sides of the hole.