I care about 5 and 10 cent differentials in price, while I and I pay for things using nickels and dimes to make exact change frequently. Whether or not the penny is rounding error doesn't matter to me,
but I don't want to round the price of everything up to the nearest quarter even if there's nothing I can buy by itself for 5 cents anymore. And I'm pretty sure I've bought things for 10 cents in recent memory.

Dollar bill and penny, sure. But please don't make the minimum price resolution of anything a quarter.