Always say "thank you" back to the people who served
you, even when they've said "thank you" to you.
It'll be nice to thank them for their service. And it makes them
happy too, to continue providing good service to the person after
you 
But don't bother saying "thank you" to those who offered you bad
service, unless it's at the end of the day, where they might be
already worn out.
