Unless I'm at the hotel for more than a week, I never let them in. Just feels weird to have strangers in my living space, however temporary it might be. I tip however, if they come in at all, mostly as a deterrent not to jack any of my shit, although I take all of my valuables with me.
No drive ins or over the counter shit generally either. In fact, a few of the places I accidentally got people in trouble, when I asked how much people would get tipped. The managers would go uh, they're not supposed to have a tip jar!
The one place that I always tip however are two pizza places in PA. Original Police Station Pizza in Ambridge, PA and D&G in Beaver Falls. It's the same type of pizza, although D&G I think is better. Anyways, it's one of square pizzas with the fluffy type of crust, and you go in, tell them what toppings you want by slice, and they do it all in their head and give you the total. I've never seen them mess up an order or an amount. It's delicious pizza, and both places have fucking amazing employees; they are never open on Sundays for example, but they opened and gave everyone discounts all day sunday until the superbowl started so people could get pizza. Often I'll take a 20+ with me, buy whatever I need and give them whatever I have leftover - which is often $10+.
That is, however, what everyone has said previously. You tip for places you love, good service etc, to make sure they stay in business and you appreciate them. Both places make you feel awesome, like they truly appreciate you. I lived by one of them at a period in time and they knew me and the orders I placed and would have it ready immediately. Great places. Shit, I want that pizza now. Curse you San fran.