I work the To Go counter at a local pizza restaurant, and we have the custom of having a tip jar in front of the register. While the servers make a server pay ($2.15/hr) and work mainly off tips, the To Go workers get regular pay and the tips are just a "bonus." I'll be honest with you -- I really don't expect people to tip. There's really no need; we honestly aren't doing anything to merit tips. I mean I love it when people tip and won't argue if you do try and tip, but I don't feel stiffed if I'm not tipped.
I will say, there are a few times I think the customer maybe should tip. Some orders are huge and complicated and I spend a good 10 minutes or so on the phone with them, then go and explain their specific order to the person making their pizzas. Then when those customers come in, they'll have us running around getting all kinds of stuff, and then usually I bring their food to the car for them. I'm not saying they SHOULD tip, but a couple bucks in the tip jar for an order for pizza over $100 would be nice.
Some people give really good tips, which I'm always surprised (as in a $5 tip on a $25 order is AMAZING). By the end of the night, I tend to go home with about $15 in tips (and I have to split the tips with my "helper"; on days I don't have a helper, I make about $25).
I've noticed I always tip to go people (at Starbucks, etc) because I know I appreciate the tips. But they're not necessary.