If you're referring to "culling" in KY ... then yes, it is legal, and yes, tourney anglers DO cull.
I do know that, in some states, if you have a partner in the boat ... you have to keep your fish separated. So that, say you catch your limit & your partner is one short of his, then YOU can't catch & keep a fish on his behalf. You'd have to release the fish or cull one of yours & put the new fish in YOUR section of the livewell.
... pappy