it has been my particular experience that you should never tell someone because you are expecting a particular response: if you are expecting they will hunt someone down, defend you, testify in court for you, feel guilty, feel angry, feel sad....then you are not ready to tell them.
if you want to tell someone because you want to give them the chance to understand you, to know you better, to give them the chance to feel closer to you....then you should.
i don't know that it will work for you, but that' been my personal experience.