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	$11.96	12.872%	$13.4995	$13.50	$13.50	$13.50
Purchase #2	$134.65	3.361%	$139.1756	$139.18	$139.20	$139.20
Purchase #3	$11.51	9.651%	$12.6208	$12.62	$12.60	$12.60
Purchase #4	$165.89	10.256%	$182.9037	$182.90	$182.90	$182.90
Purchase #5	$59.80	9.370%	$65.4033	$65.40	$65.40	$65.40
Purchase #6	$12.50	3.809%	$12.9761	$12.98	$13.00	$13.00
Purchase #7	$159.41	13.283%	$180.5844	$180.58	$180.60	$180.60
7 purchase totals =	$555.72		$607.1634	$607.16	$607.20	$607.20