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	$37.44	5.724%	$39.5831	$39.58	$39.60	$39.60
Purchase #2	$100.54	4.611%	$105.1759	$105.18	$105.20	$105.20
Purchase #3	$32.50	9.621%	$35.6268	$35.63	$35.65	$35.60
Purchase #4	$203.79	10.651%	$225.4957	$225.50	$225.50	$225.50
Purchase #5	$190.97	12.496%	$214.8336	$214.83	$214.85	$214.80
Purchase #6	$18.64	13.302%	$21.1195	$21.12	$21.10	$21.10
Purchase #7	$24.37	4.724%	$25.5212	$25.52	$25.50	$25.50
7 purchase totals =	$608.25		$667.3558	$667.36	$667.40	$667.30