Do you tip the girl working the drive through when you go to Mc Donalds? When you go to the corner party store and get a slice of pizza, do you tip the guy behind the counter? When you go to the auto parts store and the guy gets a part from behind the counter, do you tip that person?
The fact of the matter is that the people working carry at a resturant are hourly paid employees, just because its "Food" doesn't automatically make them entitled to get 10% of your entire bill.
Tips are earned, not deserved!
You sit down and eat at a resturant, the person waiting on your table has to keep checking up on your table every few minutes to check if you need anything. If you need a refill, they take care of it. If you need silverware, ketchup, napkins this is all up to your server to bring it to you. How well they perform their task depends on what you leave them.
Its like when I went out with a friend of mine, and joking with the server he took out 5 (1) dollar bills and set them on the table and said the following...
Here is $5, whenever you do something good, I add to this pile. Whenever you do something bad, I take away. Your goal is to make me keep adding to this pile :)
But I never tip the carry out person unless I feel they went above and beyond to take care of me!