If you show a winning hand to the folder, and you had them covered, it only reinforces their decision to fold, which is a psychological positive for them. Why would you want to bolster their confidence in reading you? If you bluffed rags to the river and they folded, and you show them your cards, you only make it more difficult to bluff the next time. I can think of very, very few occasions when it's a good idea to show your winning hand. I notice a lot of online players show their cards after no one calls their all-in, as if they're afraid someone will think they were "buying the pot" if they don't demostrate a strong hand. That's probably the worst reason of all.
One of the classics is last year's WSOP
$5,000 NLHE event. James Sousa caught an unbelieveable run of cards and went all-in on six straight hands. After every hand, when everyone had folded, he'd show monster holes. AA, KK, QQ, AK suited...it was freakish. He finally got into a hand against Thomas Keller, who actually went on to win the event. Keller had Sousa's rags crushed. Sousa went all-in for about half of Keller's chips. Keller thought long and hard, but folded. Sousa turned over rags. "I was on the draw." Keller looked like he was going to throw up! Beautiful.