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	$16.54	11.822%	$18.4954	$18.50	$18.50	$18.50
Purchase #2	$125.51	3.357%	$129.7234	$129.72	$129.70	$129.70
Purchase #3	$222.81	8.804%	$242.4262	$242.43	$242.45	$242.40
Purchase #4	$177.92	4.240%	$185.4638	$185.46	$185.45	$185.50
Purchase #5	$193.83	11.381%	$215.8898	$215.89	$215.90	$215.90
Purchase #6	$52.28	6.524%	$55.6907	$55.69	$55.70	$55.70
Purchase #7	$138.27	7.627%	$148.8159	$148.82	$148.80	$148.80
7 purchase totals =	$927.16		$996.5052	$996.51	$996.50	$996.50