[drostie]

It's a round, not a ceiling function. If you were going to pay .98, .99, .00, .01, or .02 you will now pay .00; whereas if you were going to pay .03, .04, .05, .06, or .07 you will now pay .05 . In other words, after the very first interval of 5 cents (about $0 even) each numeral gets 5 incoming numbers associated with it.

The only people who could really try to steal pennies from everyone would be supermarkets, and they would face the problem that it's too broadly peaked -- people going to the supermarket will buy between two and twenty things, and modulo 5 that sum is not easy to predict or steer.