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	$13.92	8.514%	$15.1051	$15.11	$15.10	$15.10
Purchase #2	$45.61	13.629%	$51.8262	$51.83	$51.85	$51.80
Purchase #3	$207.71	12.398%	$233.4619	$233.46	$233.45	$233.50
Purchase #4	$60.91	11.837%	$68.1199	$68.12	$68.10	$68.10
Purchase #5	$143.63	3.645%	$148.8653	$148.87	$148.85	$148.90
Purchase #6	$84.43	5.462%	$89.0416	$89.04	$89.05	$89.00
Purchase #7	$31.77	4.453%	$33.1847	$33.18	$33.20	$33.20
7 purchase totals =	$587.98		$639.6047	$639.61	$639.60	$639.60