I tumble with cob but I got over using polish long ago; a satin finish cleaning is all I need or want. It would be quite hard to prove that 'smoked' vs. purty cases make sizing easier or wear the dies less, or that they will shoot a bit better. Getting cases surgically clean inside is quite purty but unless you're going to do surgery with 'em it really won't matter.
The actual value of tumbling vs. hand cleaning depends on the quanity to be cleaned. Anything less than maybe 50-60 rounds (and a LOT of guys do no more than that very often) hardly benefits from tumbling but after that it does. When I have more than maybe a hundred to do I'll always tumble.
I can't tell that tiny cracks are any easier to spot on shiney cases than otherwise. Decap with a unversal decap die first, then clean/tumble and resize. IF you have any tiny cracks before sizing they become much bigger after sizing and are then very easy to spot.