I sometimes play with a quadriplegic. The dealer has to handle the cards and make bets for him. The player's cards are put on a rack where only he can see them. At the conclusion of the hand it is OK for the dealer to turn his cards over if this player tells him to. Aside from that, never. Well, when the IWTSTH rule is in effect, a dealer has to show cards to a whiner AFTER killing them. Of course if the winner of the pot is the one asking to see the hand, they shouldn't be killed, just in case the whiner will lose. ;-)