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	$242.41	12.654%	$273.0846	$273.08	$273.10	$273.10
Purchase #2	$30.51	5.783%	$32.2744	$32.27	$32.25	$32.30
Purchase #3	$48.78	13.134%	$55.1868	$55.19	$55.20	$55.20
Purchase #4	$90.94	4.325%	$94.8732	$94.87	$94.85	$94.90
Purchase #5	$41.36	10.246%	$45.5977	$45.60	$45.60	$45.60
Purchase #6	$91.46	8.805%	$99.5131	$99.51	$99.50	$99.50
Purchase #7	$36.89	3.427%	$38.1542	$38.15	$38.15	$38.20
7 purchase totals =	$582.35		$638.6840	$638.67	$638.65	$638.80