Depends on the type/brand of head how far you can straight or angle mill it.
By decked I assume you mean just to make them nice and flat?
If so he should not be worried about tolerances as you won't be close to the maxiums for that purpose. He may have to worry about compression ratio and PTV clearance depending on how much was needed to be taken off and what the heads are going on. Bout this is very doubtful.
