Assuming you're not trolling...
My stepmother wrote me a letter when I was 19 saying that she thought that I needed to deal with my sexuality - that I clearly wasn't straight. So I told her, duh bitch, I've known I was at least bi since I was 12, but I didn't think there was any reason to tell her.
Though I don't think people should have to hide who they are, I also don't always think it's necessary to scream it from the roof tops.
The only time anyone really needs to know is if, like Garnet said, you want to introduce your boyfriend to your family - assuming you feel strongly enough about that person to want to do that.
Or if your friends make gay jokes on a level where it kind of pisses you off because they're not being ironic with their gay jokes like we are, but seem to have some actual hatred for the gays - come out to those fuckers and watch their minds get blown.
Or if your mom keeps harassing you about when you're going to start giving her grandbabies. At some point you're going to have to give her "the talk."
Anyhow, it's up to you. My point was just that it makes more sense to me to tell people when it seems necessary, not just because you think you're supposed to. Since I ended up marrying a man, it was completely unnecessary to come out to my gma as bi on the basis of thinking I was supposed to come out to everyone to be true to myself (and we were very close) and I wish I hadn't because it was very painful for her (though she really instigated it and deserved it).