I haven't transitioned yet, and I'm not out to that many people (only friends and family). But if I'm interested in having a long-term relationship with someone, I think it's in both my and their best interest for them to know. In fact, I think it's a really important thing to talk about no later than when feelings start to develop and thoughts of long-term commitment begin to arise - they should know that I might transition, and I should know how they would respond and how it would affect the relationship. If we talk about it no later than when things are just beginning to get serious, then it's easier to cool things down and just be friends if it turns out to be a deal-breaker.
But it depends on the person. As a generalization, the more open-minded the person seems to be about these sorts of things, the earlier I talk to them about it. I like to broach the subject as early as possible. With some, I have to "test the waters" and slowly educate them. With others, it can be discussed more bluntly right away.
I guess I kind of have it easy because I tend to be attracted to guys who are very feminine or androgynous in terms of gender expression, are open-minded and LBGT-friendly, etc. And my relationships tend to develop naturally out of long-established friendships. Since I'm a "friends first" type of person, my preference is to open up about as much as possible when we're at that point where it's still a normal friendship but we've realized that there's potential for it to turn into something more.
