The IRS generally allows rounding to the nearest dollar on tax forms (at least individual and small business; not sure if large corporate are allowed to round even more). In fact, all the common tax software does so automatically; the only way to file a return to the cent is to do so by hand.

I'm pretty sure they don't, or else they'd be hounding you for the fractional pennies that you owe them (3/20 of every penny earned, remember!).

I doubt they care, as long as you pay 10% (or whatever it is) of total taxable revenue. I think that's how it works in Australia, where we do round.

The tax man only cares about aggregates, unless they are in audit mode.