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	$19.89	5.489%	$20.9818	$20.98	$21.00	$21.00
Purchase #2	$108.47	12.869%	$122.429	$122.43	$122.45	$122.40
Purchase #3	$30.27	13.277%	$34.2889	$34.29	$34.30	$34.30
Purchase #4	$111.40	9.411%	$121.8839	$121.88	$121.90	$121.90
Purchase #5	$188.97	11.623%	$210.934	$210.93	$210.95	$210.90
Purchase #6	$165.75	3.796%	$172.0419	$172.04	$172.05	$172.00
Purchase #7	$99.94	11.699%	$111.632	$111.63	$111.65	$111.60
7 purchase totals =	$724.69		$794.1915	$794.18	$794.30	$794.10