My husband has been in a wheelchair for over 40 years. He was hurt when he was in the Navy, in a car accident. He's a C5-C6 incomplete quadraplegic. (meaning he has some use of his hands and arms, and no use of his legs).
As for how to act when you're out and about and see a disabled person......it's really hard to say, if you don't know that person. Usually I just ask "would you like some help?" if I see that they seem to be struggling opening a door or reaching something on a shelf at the grocery store for instance. This gives them the option of saying yes or no. The important thing to remember is that they are people.....and just like able-bodied people, some of them may be kinda rude in their replies, but I always figure asking if I can help is better than just reaching over them and doing whatever for them....if they appreciate the offer they'll let you know, and if they don't want help, they'll let you know that too. Having spent a lot of time around disabled people in the last 20 years I've found that for the most part they appreciate the help, but if they don't want it, they'll let you know.
I was really kind of kidding above when I said the couple that we spent time with in Key West didn't know "wheelchair ettiquette".......but the fact is that we spent an entire day walking around with them, with me pushing Brian the entire time (on uneven sidewalks and up and down ramps to avoid stairs) and I was kinda surprised that neither one of them even said "Hey why don't you let me push him for a while?" which is what usually happens in a case like that (and what I would have done if the situation were reversed).
And YES I am glad that I've finally reached the stage where I didn't have a problem getting out of a situation we weren't enjoying!! I used to waste a lot of time and energy worrying about what others thought. I still try to consider their feelings, but in the end, I do what's right for ME!!