The same could be said for going out on that first date with someone special. How long does it take for them to realize who you really are?
You're right in that not everyone masks well, and even if they do, how much mental energy, stamina, self-discipline, and self-control are you expected to produce over the long-term?
Every situation is different. Do you tell people you are autistic so that they can put things into proper perspective, perhaps allowing you to relax a bit, or are they going to use this knowledge against you in some way? In my life, both happens, but at least I am in control of the situation. People who reject or marginalize me, can go away. They're not the people I want to associate with anyways. I can weed people out of my life quickly. Those that can accept things and still be a good, friendly, reliable, acquaintance or friend, I hang onto. As we get older and less tolerant of people, we are much quicker to accept rejection and simply move on, rather than getting upset. Young people really struggle with this, as their self-esteem is often tied to how many people like them.
I am of the mind that honesty is the best policy. Be confident. Take control and put yourself in a position of power, and never an anxious, defensive position. There are those that will accept. There are those that will reject or marginalize. Quickly get rid of the later.
This topic is really difficult with young people, especially those kids who are perceived as "different" and are subjected to bullying, teasing, marginalization, and outright rejection. Kids can be horrible to one another. How do you instill this thing called "self-confidence" and "self-esteem" in someone who seems always on the defense and is developing social anxieties because of it? "Why don't people like me?" As a parent, this sort of thing is worrisome and sad, in part, because in their world, it is such a big deal to them. The whole conversation about "Well, when you are older,....blah, blah, blah." is meaningless when they need the help now, not acceptance of their situation years from now.