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	$38.83	3.102%	$40.0345	$40.03	$40.05	$40.00
Purchase #2	$78.57	6.707%	$83.8397	$83.84	$83.85	$83.80
Purchase #3	$125.30	10.847%	$138.8913	$138.89	$138.90	$138.90
Purchase #4	$9.86	6.816%	$10.5321	$10.53	$10.55	$10.50
Purchase #5	$38.93	8.825%	$42.3656	$42.37	$42.35	$42.40
Purchase #6	$244.89	4.509%	$255.9321	$255.93	$255.95	$255.90
Purchase #7	$125.33	9.479%	$137.21	$137.21	$137.20	$137.20
7 purchase totals =	$661.71		$708.8053	$708.80	$708.85	$708.70