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	$16.78	7.263%	$17.9987	$18.00	$18.00	$18.00
Purchase #2	$149.72	10.755%	$165.8224	$165.82	$165.80	$165.80
Purchase #3	$206.88	4.426%	$216.0365	$216.04	$216.05	$216.00
Purchase #4	$11.47	11.638%	$12.8049	$12.80	$12.80	$12.80
Purchase #5	$105.93	7.700%	$114.0866	$114.09	$114.10	$114.10
Purchase #6	$114.47	11.162%	$127.2471	$127.25	$127.25	$127.20
Purchase #7	$113.76	6.269%	$120.8916	$120.89	$120.90	$120.90
7 purchase totals =	$719.01		$774.8878	$774.89	$774.90	$774.80