Here's one way to think about it. These are strangers that you will likely never see again. So, your child is really your primary responsibility and your primary audience. Starting from that idea consider these questions: Do you think it would be helpful to him to hear his behavior explained or excused due to autism? Would you want him someday to say to say to strangers that he has autism? The answer to those questions is going to depend a lot on your child and on how his autism affects him, but my main suggestion would be to evaluate your comments in terms of how they affect your son. On some level, I think people who are going to give you dirty looks or say rude stuff are going to do it anyway because a lot of people don't understand special needs kids.
I second this 100%. I never use the word "Autism" or "Asperger's" in my son's hearing, although privately we have discussed the special, different ways that his brain works (which, for example, allows him really super good night vision). The labels are not helpful because they concretize an identity. Even for children, identity formation is an enormous and complicated undertaking. The use of labels, especially as explanatory devices, just doesn't help in the long run. I don't think passing out cards is a good idea either (expensive, wasteful--- people just pitch them when they're finished reading them). The fact of the matter is, 1 in 100 kids is somewhere in the spectrum and 100 in 100 kids will have a tantrum at some point in a public place. People either gotta get used to it, or find another planet to live on because the ASD kids are going to rule the world. Breathe through your nose, smile politely, and if you need to say something to let off some tension, just tell the cashier (or ice-cream man, or unfortunate person stuck in the elevator with you) that you appreciate their patience. No explanation.
One time, when I received a really mean comment (on the order of "some children just shouldn't be allowed in public"), I looked at the lady very levelly and said, "Little Timmy didn't get his crack fix this morning and has been grumpy ever since. Can we borrow your crack pipe?" She looked shocked and said, "I don't smoke crack!" And I smiled, very nicely, and said: "Oh. My mistake. I thought you were in between highs, and that's why you were so rude." I don't honestly advocate being a wise-@ss in these situations, but it does occasionally pay to see the humor of it all.