Do you say "yes sir/no sir" (or ma'am) to employees at Taco Bell

[minionofmidas]

C) I only say "Sie", but only sometimes (and never to people in fastfood restaurants).

The common thing in Germany is to avoid if possible both the "Sie" and the "du" when talking to employees of fastfood restaurants, supermarkets, gas stations etc (unless they are fairly elderly), and if the customer is fairly young then the other way round as well. The plural serves well: "Habt ihr xy da", "kann ich mal eure Toilette benutzen", etc.