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	$19.82	7.859%	$21.3777	$21.38	$21.40	$21.40
Purchase #2	$92.99	13.421%	$105.4702	$105.47	$105.45	$105.50
Purchase #3	$204.56	12.790%	$230.7232	$230.72	$230.70	$230.70
Purchase #4	$162.62	6.349%	$172.9447	$172.94	$172.95	$172.90
Purchase #5	$26.29	10.482%	$29.0457	$29.05	$29.05	$29.00
Purchase #6	$41.62	5.541%	$43.9262	$43.93	$43.95	$43.90
Purchase #7	$34.81	8.508%	$37.7716	$37.77	$37.75	$37.80
7 purchase totals =	$582.71		$641.2593	$641.26	$641.25	$641.20