I do, and I don’t bring it up until it looks like there might be a reason for them to know. It’s for the exact opposite of the reason Gambit states, though.
If I show someone the Horrible Thing, they will probably run away, and it’s true that if you have someone who doesn’t, they’re a winner. On the other hand, most people judge by looks at the beginning of a relationship, so there’s a great chance that the Horrible Thing will be the biggest deal-breaker. On the other hand, if they get to know you and like you, there’s less of a chance that the Horrible Thing will make them run away because it’s not the biggest thing they know about you. They know your wonderful personality, sparkling intelligence, and great cooking, and oh yeah, there’s a Horrible Thing that is outweighed by these. They never know all that if the first thing they learn is that you have a Horrible Thing.
If and when it comes time to tell about the Horrible Thing, I just explain what it is, why it exists, and don’t be too shocked if I don’t look like all the other women. It’s usually shocking at first, because they’ve never seen something like that before and they’re not used to it, but a little knowledge about the Horrible Thing and about you being a wonderful person who has a Horrible Thing (and not just a Horrible Thing with a person attached) can go a long way.