I worked behind the counter for many years. I was always polite, but I really tried not to get into the fake friendliness. It can be hard when your prime directive is always to smile.
I was buying Claritin last week, and the cashier felt the need to chime in about all the luck she's been having with Zyrtec. I gave her the old blank stare. I can't stand cashiers who make comments (or worse, ask questions) about my purchases. It's unprofessional and seems like a breach of privacy and anonymity.