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	$42.31	7.580%	$45.5171	$45.52	$45.50	$45.50
Purchase #2	$79.96	12.558%	$90.0014	$90.00	$90.00	$90.00
Purchase #3	$90.90	5.800%	$96.1722	$96.17	$96.15	$96.20
Purchase #4	$18.54	3.410%	$19.1722	$19.17	$19.15	$19.20
Purchase #5	$18.72	4.212%	$19.5085	$19.51	$19.50	$19.50
Purchase #6	$216.48	10.410%	$239.0156	$239.02	$239.00	$239.00
Purchase #7	$117.61	12.413%	$132.2089	$132.21	$132.20	$132.20
7 purchase totals =	$584.52		$641.5959	$641.60	$641.50	$641.60