The commentary of pros on TV seem to say that a novice or less experienced players should not show their hand unless they are the first to act after the river. I have only shown my hand where a particular player has been especially polite or a good sportsperson. It depends on how the hand was played sometimes. If I was aggressive from the start with a powerful hand, and continue to bet out or raise, and someone calls, then folds on the river, I might show the hand to show that I play good hands strongly. Make them think....so, he doesn't bluff.