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	$19.79	6.270%	$21.0308	$21.03	$21.05	$21.00
Purchase #2	$85.65	4.522%	$89.5231	$89.52	$89.50	$89.50
Purchase #3	$103.72	6.466%	$110.4265	$110.43	$110.45	$110.40
Purchase #4	$179.77	7.408%	$193.0874	$193.09	$193.10	$193.10
Purchase #5	$223.51	7.715%	$240.7538	$240.75	$240.75	$240.80
Purchase #6	$48.89	12.816%	$55.1557	$55.16	$55.15	$55.20
Purchase #7	$56.85	9.365%	$62.174	$62.17	$62.15	$62.20
7 purchase totals =	$718.18		$772.1513	$772.15	$772.15	$772.20