Personally, I have told all of the people that I care about enough to want them to know.
If people are important to me, they've been told first hand.
The rest of the time, I wear a very visible necklace with the autistic jigsaw ribbon on. That gives me confidence, because although I'm not outright telling people and many may not have a clue what it means, I at least feel like I'm not hiding that part of me and that there's never going to be some 'big reveal'. It was hard going to friends and family and announcing that I had autism, and it felt like it was coming from nothing, but my necklace is a sort of 'middle step'. Nobody's ever commented, but there are some people that I think would know what it symbolises and may have just thought it to themselves - there may even have been one or two hints from people, to show me that they know what it is.
Only one person has ever commented outright on the necklace, and that's a stranger that turned out to immediately afterwards show some autistic traits, so I think her comment of "that's a nice necklace" was her way of saying that she was autistic. So that was lovely and opened a communication up.
To me, it's just about planting the seed so that I don't need to make a grand announcement from nothing. And, if people are thinking that something's a bit 'off' and they recognise the necklace, they can assume that's why and there doesn't need to be an awkward discussion.