You should only tell someone if you feel you need or want to, or if you feel telling the person would make them understand you or your situation better. The first person I ever told was my best friend (at the time anyway, from first grade on. I also old my mom. Other than that, the only people who know are my therapist, psychiatrist & others I've been receiving help from.
There's no need to tell someone you have SA. You might feel like you're keeping a secret or something by not telling, but, unless you feel it's in your best interest, you don't have to do it. Especially if telling could make things even harder.