I tell everyone. I do it for a few reasons. One reason is that I have a lot of false memories, and it is sometimes difficult to remember which memories are real and which are fake. If I spend tons of energy lying about something, it makes it even harder for me to remember what reality actually is. I don’t keep secrets.
Another reason is that at least once a month, someone will tell me that they have a mental illness too, and they were afraid to tell anyone because they were so ashamed. When they see me waving the metaphorical sz pride banner, it makes them realize they have nothing to be ashamed over.
Also, I know there are many MI folks who are not in positions where they can be open, due to stigma and prejudice. Folks who work in certain fields must keep quiet in order to not get fired or blacklisted. I am lucky enough that I can help bust stigma on the front lines. I have above average social skills, even for a neurotypical, I am attractive, and I am 100% nonviolent. I put a good, nonthreatening face on the illness for people in my life. Maybe, if enough people are open about their illness, the stigma will fade enough that everyone can one day be out without worrying.