If you lift a head with a steel shim gasket, the gasket stays put and the head sits back down, reseals, and everything's fine. I've taken apart motors with carbon tracks on the head surfaces where the heads had lifted in the same spot every time.
With composition gaskets, the gasket gets damaged, and sometimes the fire ring gets pushed back and bent, and then you wind up torching the deck surface.
I run steel shims when I can get them. GM still sells them, updated with a fancy plastic coating I'd just as soon not have, but hey, they're only $8.63 each.
As far as flatness of the block and deck... the embossed steel shims I've used have all been much more forgiving than those Cometic MLS gaskets so many people seem so fond of.