I agree that a map to drive displacement or bump could be used if the knurling is not of significant portion of your render or further away from the Camera where LOD is not needed. I do think though that the majority of knurling applications lends itself to put it on small items which could drive the camera lens closer to those objects (unless it is an open toolbox render). In those cases to me it would be better to actually make the geometry and then you could get as close as you need with the camera. I'm also biased by not being very great with Photoshop so the creation of a 32-bit greyscale map to drive the knurling displacement correctly would take me almost as long to just make it in MOI.