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	$18.58	7.788%	$20.027	$20.03	$20.05	$20.00
Purchase #2	$95.63	5.502%	$100.8916	$100.89	$100.90	$100.90
Purchase #3	$245.72	13.525%	$278.9536	$278.95	$278.95	$279.00
Purchase #4	$143.92	5.743%	$152.1853	$152.19	$152.20	$152.20
Purchase #5	$75.60	10.507%	$83.5433	$83.54	$83.55	$83.50
Purchase #6	$34.27	7.408%	$36.8087	$36.81	$36.80	$36.80
Purchase #7	$60.75	10.832%	$67.3304	$67.33	$67.35	$67.30
7 purchase totals =	$674.47		$739.7399	$739.74	$739.80	$739.70