I don't tell just anyone. 1st of all I do not consider myself one anymore, so there is not point in bringing it up. To those I develop relationships with ie., spouse, close friends...then yes, I do bring up my past because there are always questions regarding family. Otherwise, your past is personal, and what you were does need to be discussed unless you want to.
When I was a witness, yes I told people mainly if they asked, or if there was questions regarding holidays. It was no biggie...I had to do it, but I was never proud of it.