I've always tried to ingrain in my 3yo DS that I always expect him to be polite. This includes using his manners, not whining, listening, etc. I will remind him of this before we're going into a situation where he might be tempted not to be - like going into a nice restaurant, visiting someone else's home, going into church, etc. I feel like if he is generally polite then he's not flopping around the floor of Target because I refused to buy him a candy bar. Sometimes I will thank him for being polite or using his manners, and let him know that I really appreciate his behavior, but I don't offer a reward for it. Obviously if he isn't behaving the way I'd like him to, I deal with that according to the circumstances. Sometimes he will say, "Thank you for the chocolate milk, Mommy. Why did you get me this?" and I just tell him, "Because I know you like chocolate milk and I thought it would be a nice treat." Obviously if he isn't listening, is throwing a tantrum or something, he won't get a treat, but he's not ever guaranteed a treat if he does act nicely. I feel like it creates the atmosphere that certain behavior is just expected, and treats are just something nice that we get sometimes.
On a side note, I like the idea of "being polite" because if he is starting to get rowdy, or act inappropriately for a certain setting, I can remind him that I expect him to be polite and I don't have to go down the laundry list of things I expect out of him over and over again. He knows that it means he's supposed to listen and use his manners. It's a good trigger word for him to bring him back around again.