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	$20.37	5.612%	$21.5132	$21.51	$21.50	$21.50
Purchase #2	$205.76	10.586%	$227.5418	$227.54	$227.55	$227.50
Purchase #3	$216.53	11.680%	$241.8207	$241.82	$241.80	$241.80
Purchase #4	$111.46	6.510%	$118.716	$118.72	$118.70	$118.70
Purchase #5	$170.59	11.402%	$190.0407	$190.04	$190.05	$190.00
Purchase #6	$82.80	5.433%	$87.2985	$87.30	$87.30	$87.30
Purchase #7	$85.48	7.784%	$92.1338	$92.13	$92.15	$92.10
7 purchase totals =	$892.99		$979.0647	$979.06	$979.05	$978.90