yeah, I was definitely going to get something very flat and hard, and sand at right angles. That will keep it flat enough. I this is a deep dish piston...the guench area on the piston is actually quite small and only on one side.
If I was talking about a sealing surface or something that makes contact like that, I'd never try to sand it. But honestly, especially since I'm talking about removing a few thousandths...it would be difficult to do anything that would be very detrimental. Epecially compared to letting the piston slap the head...