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	$56.39	6.266%	$59.9234	$59.92	$59.90	$59.90
Purchase #2	$248.59	3.689%	$257.7605	$257.76	$257.75	$257.80
Purchase #3	$41.69	9.411%	$45.6134	$45.61	$45.60	$45.60
Purchase #4	$66.33	4.795%	$69.5105	$69.51	$69.50	$69.50
Purchase #5	$16.36	13.433%	$18.5576	$18.56	$18.55	$18.60
Purchase #6	$172.32	4.561%	$180.1795	$180.18	$180.20	$180.20
Purchase #7	$11.60	6.501%	$12.3541	$12.35	$12.35	$12.40
7 purchase totals =	$613.28		$643.8990	$643.89	$643.85	$644.00