I dont like telling people that dont need to know, as i find they dont actually understand what Bipolar is and then they start to look for the traits in you and anything out of character worries them.
I tell the people who i know i can trust.
I found myself talking to a nurse the other night who is a friend of a friend and she understood the illness well, and i just blurted out loads of stuff to her, just got it off my chest, it can be hard sometimes to talk to the people you know the most.
But as a rule, i dont like to sit down and say to someone, "hi, i'm bipolar."
If you're spending a lot of time with this person i would tell them though, i get people who get to know me in one episode and dont like me in another, then if there is a dramatic change they wont be so shocked.