The reason supermarkets offered cash back was that this allowed them to massively reduce the amount of cash held on site and thus reduce their insurance costs.
But wouldn’t this increase the amount of cash you need on hand to ensure you can make change and now give cash back?
I’m not sure they even offer cash back these days, it has been so long since I asked, and the number of cash transactions has dropped massively over the last few years.
Anything under 10 cents gets rounded down/up accordingly
It doesn't matter. Minting and transporting it, even in your pocket costs more than you'll ever get in dealing a more "precise" denomination
The US used to have a half-penny coin. It stopped being minted in 1857.
The purchasing power of the half-penny at the time was about as much as a quarter has today.
It wouldn't be crazy to get rid of all coins worth less than 25¢ and round all purchases to the nearest quarter.
Obligatory CGP Grey: https://youtu.be/y5UT04p5f7U