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	$18.89	13.165%	$21.3769	$21.38	$21.40	$21.40
Purchase #2	$132.98	12.332%	$149.3791	$149.38	$149.40	$149.40
Purchase #3	$95.72	9.392%	$104.71	$104.71	$104.70	$104.70
Purchase #4	$17.57	9.169%	$19.181	$19.18	$19.20	$19.20
Purchase #5	$137.89	9.859%	$151.4846	$151.48	$151.50	$151.50
Purchase #6	$17.32	10.618%	$19.159	$19.16	$19.15	$19.20
Purchase #7	$50.78	4.489%	$53.0595	$53.06	$53.05	$53.10
7 purchase totals =	$471.15		$518.3501	$518.35	$518.40	$518.50