I have BPD, and I would not disclose it first thing (I might look for therapists who treat it), but I think that it is the kind of thing that you should get disclose when you want. It can be part of therapy to trust the T enough to disclose such a thing, and I don't think that all T's will see it as relevant or necessary to focus on a DX in treatment. What matters most is the person who is there, and the way that they feel.