I think it depends on the situation. I feel like I have to tell someone before it becomes terribly serious, because I feel like they have the right to know. Also I have scars from self harm and it would be really awkward if I didn't explain before hand and then they were noticed in.. uh, certain situations.

Having said that, I wouldn't mention it before I thought things were getting to the potentially serious stage, and even then I probably wouldn't leap straight in with my diagnosis, I would say that I have / had MH problems and explain how they've affected me.