If you have a curved face fine slipstone, one that has a curvature of less than the spindles internal diameter, just give it a couple of strokes to see if it shows up any high spots, if it does, gently stone them away till it is smooth again small area of the bore that is low makes no difference, buy if part of it is high it will not seat square. We are talking about doing the absolute minimum to remove any high spots, that is all!