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	$20.89	3.342%	$21.5881	$21.59	$21.60	$21.60
Purchase #2	$203.96	8.456%	$221.2069	$221.21	$221.20	$221.20
Purchase #3	$202.60	6.813%	$216.4031	$216.40	$216.40	$216.40
Purchase #4	$79.64	10.130%	$87.7075	$87.71	$87.70	$87.70
Purchase #5	$80.51	11.332%	$89.6334	$89.63	$89.65	$89.60
Purchase #6	$234.92	8.608%	$255.1419	$255.14	$255.15	$255.10
Purchase #7	$83.76	6.362%	$89.0888	$89.09	$89.10	$89.10
7 purchase totals =	$906.28		$980.7697	$980.77	$980.80	$980.70