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	$36.83	7.507%	$39.5948	$39.59	$39.60	$39.60
Purchase #2	$88.45	11.738%	$98.8323	$98.83	$98.85	$98.80
Purchase #3	$10.90	5.190%	$11.4657	$11.47	$11.45	$11.50
Purchase #4	$37.34	13.798%	$42.4922	$42.49	$42.50	$42.50
Purchase #5	$181.45	10.720%	$200.9014	$200.90	$200.90	$200.90
Purchase #6	$40.35	13.203%	$45.6774	$45.68	$45.70	$45.70
Purchase #7	$203.67	8.289%	$220.5522	$220.55	$220.55	$220.60
7 purchase totals =	$598.99		$659.5160	$659.51	$659.55	$659.60