It used to be that they gave you the choice. You could round or you could use pennies but you had to be consistent throughout the return, because even the IRS doesn’t care if you manage to scrape out 49 cents.
It is still a choice, though I cannot remember the last time we used cents on ours or any other returns (my wife works for an accounting firm so they handle a fair number of returns). Just has to be consistent, either you round or you do not.
Not in all cases. The IRS does not use cents when you file your tax return, they say round to the nearest dollar.