Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$11.82	8.119%	$12.7797	$12.78	$12.80	$12.80
Purchase #2	$56.51	5.387%	$59.5542	$59.55	$59.55	$59.60
Purchase #3	$118.55	12.670%	$133.5703	$133.57	$133.55	$133.60
Purchase #4	$11.85	9.283%	$12.95	$12.95	$12.95	$13.00
Purchase #5	$40.25	7.256%	$43.1705	$43.17	$43.15	$43.20
Purchase #6	$148.39	10.837%	$164.471	$164.47	$164.45	$164.50
Purchase #7	$231.87	3.185%	$239.2551	$239.26	$239.25	$239.30
7 purchase totals =	$619.24		$665.7508	$665.75	$665.70	$666.00