I don't usually tip when I pick up my food indoors, with the exception of an ice cream shop in a resort town we visit regularly. It is staffed for the summer by college kids and I always leave them a dollar.
RadioNate's post got me thinking - why do we tip bartenders when we are sitting at the bar but not someone at an ice cream shop? It takes about the same amount of effort to make an ice cream sundae as it does to mix most drinks.
Because they are established tipped positions, their wages are lower, and they are taxed on assumed tips.