I used to never tip for "counter service", but then I started watching some of the ladies at the joint we pick up tacos from on most saturdays, and they're humping it just as much as any waitress, if not actually waitressing in between getting your order ready and ringing you up for it.. They answer the phones, pack the orders, bring them to the counter and ring you up. Leave there, pour someone's coffee. This is not normal at all places, obviously, but if I can't be sure, I go ahead and tip a couple of bucks.
What bugs me is a few weeks ago our group of 6 was charged gratuity without even knowing it. We all had a few drinks, so I wasn't adding everything up, and just assumed the drinks were expensive. After we already paid, one of the guys (the one not drinking, obviously) asked to see the actual ticket out of curiosity, and the gratuity was on it. After we had all tipped regularly. I don't mind it if it's the regular ticket with everything itemized, including gratuity, but we were just given the total and tax when we paid. They got us good.