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	$39.50	6.776%	$42.1765	$42.18	$42.20	$42.20
Purchase #2	$189.52	10.161%	$208.7771	$208.78	$208.80	$208.80
Purchase #3	$82.27	6.450%	$87.5764	$87.58	$87.60	$87.60
Purchase #4	$22.58	3.594%	$23.3915	$23.39	$23.40	$23.40
Purchase #5	$145.90	8.496%	$158.2957	$158.30	$158.30	$158.30
Purchase #6	$48.74	7.161%	$52.2303	$52.23	$52.25	$52.20
Purchase #7	$84.27	7.308%	$90.4285	$90.43	$90.45	$90.40
7 purchase totals =	$612.78		$662.8760	$662.89	$663.00	$662.90