I guess the main reason to tell would be if you felt it would benefit you. For instance - I'll share it with people if I feel the need to talk about issues relating to my bisexuality, and I think those people would give good feedback.

Also, for some people sexuality is a large part of their self-identity, and not mentioning it makes them feel like they're hiding 'who they are.' I don't feel this way, but I also notice I am pretty closeted in real life. I won't deny it to friends but family and strangers certainly don't get to know. Heaven forbid I'd let a coworker or professor find out. It's a very bigoted society and discrimination against homosexuals is still largely legal, with people often getting away with it even when it isn't. I was especially closed off when I was a teenager and worried coming out to my parents would make my home life worse. I also didn't see any reason to cause unnecessary drama, when chances were almost equal that I'd end up with a man anyways.

However, I find that I still tell little white lies about my life in order to maintain the illusion that their daughter could still be straight. (Mostly because I've tried to broach the topic and can tell my mother just does not want to hear it. My dad hates homosexuals and they both also feel that bisexuality doesn't exist.) So when people don't know, you almost feel it necessary to hide. Coming out can take the edge off here. They know. There's no reason to keep gender ambiguous in a sentence or not share some information. You don't have to worry so much that a small slip-up will start a giant bit of prying into your personal life, or ruin the conversation. Yes, there can be other problems which go along with coming out, and I wouldn't recommend it to teenagers who have very prejudiced parents, but there are also major negatives to not coming out which I think should also be evaluated.

I'll probably come out to my sister when she comes home for the summer. At this point she almost has a sister-in-law, and I see no reason to keep this from her. My other sister's only ten, but she knows I've had female crushes. I don't share any more than that with her, due to the fact that she'd easily slip up to my dad. My mom probably knows but she'd never admit it, because she doesn't want to believe it. And my dad will probably go to his grave never knowing. That man can be abusive and I'm not giving him any more ammunition. Oh, my brother was the first family member I told though. When my current lover came on a family vacation with us, I needed some advice and knew he could give it. So that's how that came to be. As you can see, it's depended a lot on the people in my life and situations, as well as how comfortable I feel with them knowing.

So I suppose in the end, it's a really personal decision and up to you. The annoying thing is once they know, they can't un-know. So I'd definitely test the waters before jumping in.