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	$35.85	11.534%	$39.9849	$39.98	$40.00	$40.00
Purchase #2	$37.41	12.738%	$42.1753	$42.18	$42.20	$42.20
Purchase #3	$227.96	12.685%	$256.8767	$256.88	$256.90	$256.90
Purchase #4	$75.76	4.847%	$79.4321	$79.43	$79.45	$79.40
Purchase #5	$71.59	3.494%	$74.0914	$74.09	$74.10	$74.10
Purchase #6	$132.64	11.691%	$148.1469	$148.15	$148.15	$148.10
Purchase #7	$18.91	4.591%	$19.7782	$19.78	$19.80	$19.80
7 purchase totals =	$600.12		$660.4855	$660.49	$660.60	$660.50