Most of the time I let them figure it out, if they didn't already know prior.
What i've found is that some people have preconceived notions of someone who's autistic or has AS thanks to the media, and once I tell some people, I notice they act differently (usually they act overly nicer to me, which I find discomforting) from how they did before. Most people I know that I haven't told/didn't know before, just think i'm a little bit weird or eccentric, which doesn't affect their notions of me and i'm far better off with this than them suddenly acting differently towards me.
Most of my closest friends, though, when I tell them/knew already, don't really care and don't act any different towards me and some of them have even forgotten.
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here be dragons