(NSFW) When do you tell someone about your sexual "difference"?

This is inspired by another question. Someone said a pre-op transsexual should let it be known when someone comes on to her.

So that got me thinking… when do you think someone with a sexual “difference” (not sure how else to phrase it) should tell a potential partner?

Also, does it matter what the difference is? Would your answer be different based on sexual past (“Just so you know, I slept with your brother. And your ex-wife. And the bartender.” or “I’m a virgin.”), unusual body parts (“I have a micro/gigantic penis,” “I have a very big clit,” or “I have a small tail”), or kinks (“I can only climax if your toe is in my mouth”)? What about blatant false advertising (“That’s really a sock in my pants” or “I’m wearing a megapadded push up bra, they’re actually really small.”)

First hand accounts of when you tell or when you would want to be told are fine.