Knife edging an NSR crank would lose crank inertia, which might make it sound good on the stand, but it might make it slower down the chute. It would also increase case (free) volume, so would reduce pumping efficiency.

Some HRC kit cranks (RS250 for example) are made from tungsten and although they are smaller in diameter than the standard crank, they weight more and have a higher inertia. The smaller crank much be used with kit cases, which have a smaller crank case housing. All this help reduce case volume to hit the right ratio.

If the crank is well trued, and has the perfect balance factor, then you can run less than 1mm clearance without touching.

On RS125's in Japan we were skimming the OD of the crank and bushing the cases for less than 1mm clearance, with no touching.

The lighter crank went very well at short, point and squirt tracks, but a stock crank with bushed cases was the choice at the fast tracks.