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	$8.72	13.594%	$9.9054	$9.91	$9.90	$9.90
Purchase #2	$76.34	9.245%	$83.3976	$83.40	$83.40	$83.40
Purchase #3	$146.55	11.810%	$163.8576	$163.86	$163.85	$163.90
Purchase #4	$222.29	10.466%	$245.5549	$245.55	$245.55	$245.60
Purchase #5	$157.78	4.575%	$164.9984	$165.00	$165.00	$165.00
Purchase #6	$135.93	7.741%	$146.4523	$146.45	$146.45	$146.50
Purchase #7	$82.96	9.146%	$90.5475	$90.55	$90.55	$90.50
7 purchase totals =	$830.57		$904.7137	$904.72	$904.70	$904.80