But there would be rounding down, so how is this relevant?

What's even to say anything will be rounded down? If Walmart says "we're going to round anything from $0.01 to $0.04 up to $0.05," do you think the free market would put them out of business out of principle, or would they get away with it? I think they'd get away with it.

Nobody has to round down. There's no government rule.

I would expect many businesses to implement ceil()-flavored rounding.