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	$34.97	13.810%	$39.7994	$39.80	$39.80	$39.80
Purchase #2	$26.89	10.540%	$29.7242	$29.72	$29.70	$29.70
Purchase #3	$223.57	13.587%	$253.9465	$253.95	$253.95	$253.90
Purchase #4	$112.31	4.300%	$117.1393	$117.14	$117.15	$117.10
Purchase #5	$76.97	8.785%	$83.7318	$83.73	$83.75	$83.70
Purchase #6	$188.63	6.371%	$200.6476	$200.65	$200.65	$200.60
Purchase #7	$79.55	13.290%	$90.1222	$90.12	$90.10	$90.10
7 purchase totals =	$742.89		$815.1110	$815.11	$815.10	$814.90