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	$25.84	6.552%	$27.533	$27.53	$27.55	$27.50
Purchase #2	$232.52	10.345%	$256.5742	$256.57	$256.55	$256.60
Purchase #3	$229.69	11.590%	$256.3111	$256.31	$256.30	$256.30
Purchase #4	$103.65	7.563%	$111.489	$111.49	$111.50	$111.50
Purchase #5	$58.40	5.386%	$61.5454	$61.55	$61.55	$61.50
Purchase #6	$9.54	10.547%	$10.5462	$10.55	$10.55	$10.50
Purchase #7	$231.29	6.401%	$246.0949	$246.09	$246.10	$246.10
7 purchase totals =	$890.93		$970.0938	$970.09	$970.10	$970.00