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	$16.75	10.302%	$18.4756	$18.48	$18.50	$18.50
Purchase #2	$53.74	7.513%	$57.7775	$57.78	$57.80	$57.80
Purchase #3	$93.74	10.802%	$103.8658	$103.87	$103.85	$103.90
Purchase #4	$125.74	12.291%	$141.1947	$141.19	$141.20	$141.20
Purchase #5	$30.73	9.536%	$33.6604	$33.66	$33.65	$33.70
Purchase #6	$105.59	13.196%	$119.5237	$119.52	$119.50	$119.50
Purchase #7	$33.58	7.546%	$36.1139	$36.11	$36.10	$36.10
7 purchase totals =	$459.87		$510.6116	$510.61	$510.60	$510.70