For reasons of fairness, that should be "rounded" rather than "rounded
up".

Nope!

The consistent (and in the long run most "fair") is to remember that when
you now buy ONE of something that's "Three for a $dollar" the first item is
$.34. The next two are only $.33.

So long as everyone plays by the same rules and everyone understands the
rules "fairness" isn't an issue.