If they truly are "mirror finish", then the surface is perfectly uniform, or on the same level. If they get scratched, you can polish out that one area in order to make the scratch disappear, but then that particular area will have a slight dish to it. It would be barely perceptible, but the only way to make it absolutely perfect again would be to sand the wheel, like by lifting the car up and using sandpaper on a rubber block and then having someone else spin the wheel while you polish it, like a lathe or a potter's wheel, to achieve a truly uniform finish. Preferably, you would remove the wheel and put it on something like a spin balance machine to do this. You of course follow with the finest polish. THAT is how you get perfection. Of course, you have to take into consideration the fact that gravity tends to ever so slightly cause the bottom end of things to droop down, so you will need to hitch a ride into outer space to do this in zero G. That is how you make the most perfect bearings. At what point do you stop striving for perfection and accept a practical approach?