Jaded or Normal?

I had to stop for gas on the way home tonight (no complaints; I'd gone six days between fill-ups). Having some extra cash in my pocket (thanks to the combination of a student paying me that way, and the fancy new ATM not accepting my bills), I decided to pay for my gas in cash. It had been years since I'd done this (save for the occasional top-off), and I was trying to remember the most efficient way to do this. The following conversation resulted:

ME: OK, I want to fill up on Pump #1. Do I just leave you a deposit? (pulling out a twenty as i say this)
STORE CLERK: That's gonna stop at twenty if you do that.
ME: Hmm...it might be a bit more than twenty, I'm not sure. I can't predict it down to the dime. How should I work this?
CLERK: You give me more than you think you'll need; that way, you don't drive off and leave me hanging.
ME (laughing): Oh, there's no way I'd ever do that.
CLERK (frowning): I don't know that; I don't know you.
ME: Yeah, I guess you could get kinda jaded in a job like this.

Afterwards, I realized that this exchange left me with a whole bunch of questions:

1) Wasn't there a time when you left a certain amount of cash to fill up as sort of a "deposit," and then, when you came back in, you either paid the balance or received the change? Or am I just imagining that this used to happen?

2) Was the store clerk in any way being a jerk for seeming to automatically brand every customer as a potential drive-off, or is that just part of their training?

3) And was he really jaded by his job, or is that normal in retail these days? (I've been out of the loop for a while.)

Feel free to share your opinion in the comments.