Question: Tip for Carryout?

[spooble]

I tip for carryout on occasion, but never at fastfood joints. One place I can never decide if I should tip or not is Sonic Drive-In. I mean.... it's a fastfoot place... but they do walk it out to your car. (Or sometimes roller skate). I've just been rounding up to the next dollar, but I wonder if they're cussing me as they skate back inside.