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	$11.52	13.729%	$13.1016	$13.10	$13.10	$13.10
Purchase #2	$248.69	10.633%	$275.1332	$275.13	$275.15	$275.10
Purchase #3	$109.40	13.530%	$124.2018	$124.20	$124.20	$124.20
Purchase #4	$234.56	8.494%	$254.4835	$254.48	$254.50	$254.50
Purchase #5	$7.65	11.257%	$8.5112	$8.51	$8.50	$8.50
Purchase #6	$54.98	8.822%	$59.8303	$59.83	$59.85	$59.80
Purchase #7	$51.29	6.319%	$54.531	$54.53	$54.55	$54.50
7 purchase totals =	$718.09		$789.7926	$789.78	$789.85	$789.70