I never show my cards, as I have the auto-muck option checked at all times.
On sites where you can disable this easily at the table (like ACR), then I will sometimes show my cards if play gets folded to me in the BB and I either have a monster (QQ+/AK) or absolute garbage (72o and the likes). It gives out no information about my play whatsoever (since it just got folded to me) but if I show a monster like this, it might make the others think I'm either tilting or less likely to get a good hand shortly after. And showing garbage might just tilt the BTN and SB.
When anyone asks what I had, my response is "now you'll never know".
The exception is during a CC Freeroll. I'll honestly say what I had when asked there, and occasionally reassure someone that they made a correct fold, or that I laid down a big hand.