In my experience, the likelihood of peening the case mouths depends on a few factors:
- Time - the longer you tumble, the more likely you are to damage the mouths
- Fill - the more full the tumbler is, the less the cases 'fall' as they get carried up the side of the drum as it rotates. The less they fall, the less impact damage to the mouths.
- Size - the bigger the cases, the more mass they have as they fall, and the more damage they do to surrounding cases when they hit.
As an example... a Thumler's Tumbler with a couple hundred .223 Rem or 5.56 cases can go for hours or even overnight without needing anything more than a routine trim/chamfer/debur like you'd normally do. The same tumbler with about fifty .338LM cases might ruin them all beyond salvation if you tumble them for a couple hours.
In my experience, they're about as clean as they're going to realistically get after about 45 minutes. Tumbling longer to get that last little bit out of the primer pocket ain't worth the potential problems.
FWIW... I - and most of the better long-range shooters I know - have stopped doing wet tumbling (or ultrasonic) for match brass. If you strip the insides of the necks back down to bare metal, you end up with excessive and/or inconsistent seating force. It is possible to put some sort of lube back inside the case necks, either using ceramic beads or a soft bore mop loaded with dry graphite or moly powder, but why bother? I'm largely back to tumbling with dry media, and brushing the insides of the necks on the 'good' brass to make sure they are clean/clear/consistent.
YMMV.