You are taking someone's order at a fast food place. If they are being rude, do you be rude back or keep professional?
As with any other professional setting, I don't deviate.
I work in a restaurant. I frequently say that I would give anything to have just one day where I could say what I want to say to customers, without any negative repercussions. Alas, this immunity doesn't exist, so I smile and keep going.