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	$16.62	8.288%	$17.9975	$18.00	$18.00	$18.00
Purchase #2	$132.47	4.135%	$137.9476	$137.95	$137.95	$137.90
Purchase #3	$227.59	12.405%	$255.8225	$255.82	$255.80	$255.80
Purchase #4	$135.29	13.769%	$153.9181	$153.92	$153.90	$153.90
Purchase #5	$142.65	10.316%	$157.3658	$157.37	$157.35	$157.40
Purchase #6	$52.81	5.256%	$55.5857	$55.59	$55.60	$55.60
Purchase #7	$54.29	12.122%	$60.871	$60.87	$60.85	$60.90
7 purchase totals =	$761.72		$839.5082	$839.52	$839.45	$839.50