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	11.354%	$21.057	$21.06	$21.05	$21.10
Purchase #2	$180.45	12.739%	$203.4375	$203.44	$203.45	$203.40
Purchase #3	$89.52	6.605%	$95.4328	$95.43	$95.45	$95.40
Purchase #4	$111.25	13.265%	$126.0073	$126.01	$126.00	$126.00
Purchase #5	$243.91	7.549%	$262.3228	$262.32	$262.30	$262.30
Purchase #6	$30.96	10.633%	$34.252	$34.25	$34.25	$34.30
Purchase #7	$52.96	12.510%	$59.5853	$59.59	$59.60	$59.60
7 purchase totals =	$727.96		$802.0947	$802.10	$802.10	$802.10