I remember thinking there's a small arbitrage opportunity in countries that don't have pennies and nickels. In NZ I believe stores round to the nearest decimal (.02, .01 => .00 and .03, .04 => .05). They lose on some sales but gain on others. However, they don't round if you use a CC.

Here's one for the FIRE folks: if it rounds up, use a CC and if it rounds down use cash. Use all those pennies you save using your CC to retire 3 minutes early.