Good question. I think it's player dependent. If it's a really good player, I'll never show. I want them to keep thinking, but to never be REALLY sure of what I am doing.
If it's not as good a player, I'll show them a bluff or the nuts. Why? Well, here's one example. In a bar league years ago, there was an older couple who would play. The husband was a decent enough player, and his wife wasn't a bad player either, but he would mix it up more than she would, whereas she would play premium hands. When those premium hands would lose, she would berate you, saying things like "I can't believe you played those two cards when I had AK", something like that.
With her, I'll show my cards the odd time to get into her head. The thing too with her was she would just start to spout off when her better pre-flop hands would lose so why not feed the fire some more? If I felt I had an advantage, I would take it against that particular opponent.