Here in Argentina the law says they must be rounded down. Initially it was for 5 AR$cents, and some shops still has the oficial sign that says AR$ 0.05.

We unofficially drop the coins/bills when the reach ~US$0.03, so now we dropped the AR$50 bills and everythig in cash is rounded down to AR$100 (US$0.07).

(The only exception is the photocopy shop 2 blocks away from home.)

Credit cards are charged the exact ammount, with cents that are irrelevant.