[frankus]

Rounding to a nickel has the advantage that's it's both simple (1, 2, 6, and 7 round down; 3, 4, 8, and 9 round up) and fair (there's no systematic bias in favor of the buyer or seller).

Dimes need to deal with how to round numbers ending in 5, making them unfair, or (with a more elaborate system of looking at both digits) complicated.

Quarters (being an odd value) are fair, but kind of a nightmare to memorize all of the values that round up or down (1–12, 26–37, 51–62, and 76–87 round down; 13–24, 38–49, 63–74, and 88–99 round up; and I'm not even sure I don't have an error in those numbers).

Also it's awkward if a higher-valued coin (e.g. quarters) isn't divisible by the least valuable coin (e.g. a dime)