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	$18.50	10.801%	$20.4982	$20.50	$20.50	$20.50
Purchase #2	$39.77	5.738%	$42.052	$42.05	$42.05	$42.10
Purchase #3	$34.61	11.208%	$38.4891	$38.49	$38.50	$38.50
Purchase #4	$36.91	9.444%	$40.3958	$40.40	$40.40	$40.40
Purchase #5	$22.49	7.278%	$24.1268	$24.13	$24.15	$24.10
Purchase #6	$213.89	13.117%	$241.946	$241.95	$241.95	$241.90
Purchase #7	$105.36	7.655%	$113.4253	$113.43	$113.45	$113.40
7 purchase totals =	$471.53		$520.9332	$520.95	$521.00	$520.90