DS had a "diagnosis" of mild to moderate autism by the school psychologist (in quotes because I don't know that she was actually qualified to make such a diagnosis). I regret telling my close friends and family about this, especially as a few months later a private psychiatrist told us that yes, DS has some quirks, but she couldn't even call it Aspergers, let alone autism.
I wish I had waited, or held off entirely, as some reactions were not particularly positive. But because having a 2e kid is so isolating, I feel compelled to tell my family a lot of stuff just to get it off my chest, iykwim.
And while it's true that a diagnosis or a label doesn't change who DS is, it can change how people interact with him, and not always for the better. Once you release information to the world, there's no bringing it back.