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	$18.91	11.308%	$21.0483	$21.05	$21.05	$21.00
Purchase #2	$25.41	5.204%	$26.7323	$26.73	$26.75	$26.70
Purchase #3	$116.67	9.844%	$128.155	$128.16	$128.15	$128.20
Purchase #4	$75.43	4.155%	$78.5641	$78.56	$78.55	$78.60
Purchase #5	$40.65	4.130%	$42.3288	$42.33	$42.35	$42.30
Purchase #6	$228.65	11.497%	$254.9379	$254.94	$254.95	$254.90
Purchase #7	$54.44	11.171%	$60.5215	$60.52	$60.50	$60.50
7 purchase totals =	$560.16		$612.2879	$612.29	$612.30	$612.20