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	$18.75	12.366%	$21.0686	$21.07	$21.05	$21.10
Purchase #2	$97.50	8.259%	$105.5525	$105.55	$105.55	$105.60
Purchase #3	$199.50	11.323%	$222.0894	$222.09	$222.10	$222.10
Purchase #4	$88.55	11.205%	$98.472	$98.47	$98.45	$98.50
Purchase #5	$178.77	6.583%	$190.5384	$190.54	$190.55	$190.50
Purchase #6	$143.30	7.546%	$154.1134	$154.11	$154.10	$154.10
Purchase #7	$15.81	12.454%	$17.779	$17.78	$17.80	$17.80
7 purchase totals =	$742.18		$809.6133	$809.61	$809.60	$809.70