Regardless of how GM does it, you won't be able to rehone your block without going oversize in some way. The block was honed to fit your pistons, then the rings wore it larger, then honing it again will make it bigger yet.
The deck should be checked for flatness, but if you don't need to shave it, don't. No need to make the decks thinner if they don't need to be... just like you're not anxious to overbore unless you need to. With flat tops .020 in the hole plus your .015 gasket will make a nice tight quench.
Most good shops wait for the pistons to come in so they can measure that specific set and do final honing for your pistons.