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	$18.83	12.256%	$21.1378	$21.14	$21.15	$21.10
Purchase #2	$59.82	12.766%	$67.4566	$67.46	$67.45	$67.50
Purchase #3	$123.47	13.429%	$140.0508	$140.05	$140.05	$140.10
Purchase #4	$111.63	7.338%	$119.8214	$119.82	$119.80	$119.80
Purchase #5	$73.49	6.817%	$78.4998	$78.50	$78.50	$78.50
Purchase #6	$26.94	5.355%	$28.3826	$28.38	$28.40	$28.40
Purchase #7	$247.48	6.147%	$262.6926	$262.69	$262.70	$262.70
7 purchase totals =	$661.66		$718.0416	$718.04	$718.05	$718.10