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	$5.89	12.171%	$6.6069	$6.61	$6.60	$6.60
Purchase #2	$91.49	7.726%	$98.5585	$98.56	$98.55	$98.60
Purchase #3	$155.52	10.591%	$171.9911	$171.99	$172.00	$172.00
Purchase #4	$96.77	3.241%	$99.9063	$99.91	$99.90	$99.90
Purchase #5	$39.42	8.792%	$42.8858	$42.89	$42.90	$42.90
Purchase #6	$17.90	6.246%	$19.018	$19.02	$19.00	$19.00
Purchase #7	$35.25	13.352%	$39.9566	$39.96	$39.95	$40.00
7 purchase totals =	$442.24		$478.9232	$478.94	$478.90	$479.00