What is the correct etiquette for tipping when you get carry-out?

[lexsmoma]

I went to two different places in the last week, Quizno's and Chili's, for take-out for my family. At each counter there was a tip jar. I was just wondering what is expected of you when you get carry-out. Are they expecting a tip? Is it optional? Would the change from your bill be an insult? or appreciated? I guess I never really thought about it because I wouldn't tip a drive through worker and they do the same amount of work as the person who puts my food in a bag and rings me up. Also, I am a cashier at a grocery store who does about the same amount of labor per person as a carry-out worker would, but I don't expect a tip. Is it because waitstaff makes so little per hour? Just curious what everyone thought and what everyone does in this situation. I'd love to hear from customers AND employees if possible. Thanks!