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	$16.99	6.763%	$18.139	$18.14	$18.15	$18.10
Purchase #2	$39.82	13.841%	$45.3315	$45.33	$45.35	$45.30
Purchase #3	$143.27	5.224%	$150.7544	$150.75	$150.75	$150.80
Purchase #4	$207.55	4.682%	$217.2675	$217.27	$217.25	$217.30
Purchase #5	$100.97	3.600%	$104.6049	$104.60	$104.60	$104.60
Purchase #6	$107.64	8.659%	$116.9605	$116.96	$116.95	$117.00
Purchase #7	$55.98	12.272%	$62.8499	$62.85	$62.85	$62.80
7 purchase totals =	$672.22		$715.9077	$715.90	$715.90	$715.90