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	$28.63	6.361%	$30.4512	$30.45	$30.45	$30.50
Purchase #2	$242.38	7.445%	$260.4252	$260.43	$260.45	$260.40
Purchase #3	$16.53	11.193%	$18.3802	$18.38	$18.40	$18.40
Purchase #4	$87.81	8.475%	$95.2519	$95.25	$95.25	$95.30
Purchase #5	$34.67	9.376%	$37.9207	$37.92	$37.90	$37.90
Purchase #6	$50.95	7.658%	$54.8518	$54.85	$54.85	$54.90
Purchase #7	$132.93	8.342%	$144.019	$144.02	$144.00	$144.00
7 purchase totals =	$593.90		$641.3000	$641.30	$641.30	$641.40