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	$5.58	13.340%	$6.3244	$6.32	$6.30	$6.30
Purchase #2	$36.93	7.576%	$39.7278	$39.73	$39.75	$39.70
Purchase #3	$26.82	4.203%	$27.9472	$27.95	$27.95	$27.90
Purchase #4	$116.32	10.518%	$128.5545	$128.55	$128.55	$128.60
Purchase #5	$62.59	12.677%	$70.5245	$70.52	$70.50	$70.50
Purchase #6	$55.32	11.125%	$61.4744	$61.47	$61.45	$61.50
Purchase #7	$179.35	3.275%	$185.2237	$185.22	$185.20	$185.20
7 purchase totals =	$482.91		$519.7765	$519.76	$519.70	$519.70