Just my feelings, when someone puts something, anything in my hand, I always say thank you - just me, just my habbit. Truthfully, if I am polite enough to HAND someone something, they should be polite enough the say thanks, I mean, its not like its much effort.
And my automatic response when someone hands me cash to pay for something is to say "out of $x". I just told them the total, they hand me money, I verify the amount with them, count change and then say "thank you." If I say thank you when I'm handed the cash then it delays the whole transaction, or slurs together with the confirmation "out of $20, thank you..." doesn't really give them time to respond/correct before I have the change in my hands.
Not that efficiency is the most important thing in a transaction, but I learned my money handling habits in a busy ice cream store. Moving the line and cheerful smiles/conversation made everyone a lot happier than many automatic "thank you"s. Someplace where transactions were more leisurely and personal, things might be different.
Her cousin was maybe 14-15. I very nicely let her know that we would have to call the police if her cousin was checked back in and had been drinking. She said "I was joking! Geez!", snarled, and was gone. We all just looked at each other. The other parents in the office at that time were shocked! I can understand some parents joking about that since we're at a college and all, but we were not having any type of pleasant conversation. It was so weird....