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	$6.39	4.598%	$6.6838	$6.68	$6.70	$6.70
Purchase #2	$35.98	11.867%	$40.2497	$40.25	$40.25	$40.20
Purchase #3	$53.88	4.811%	$56.4722	$56.47	$56.45	$56.50
Purchase #4	$133.47	7.406%	$143.3548	$143.35	$143.35	$143.40
Purchase #5	$54.81	9.118%	$59.8076	$59.81	$59.80	$59.80
Purchase #6	$12.69	5.239%	$13.3548	$13.35	$13.35	$13.40
Purchase #7	$178.52	7.809%	$192.4606	$192.46	$192.45	$192.50
7 purchase totals =	$475.74		$512.3835	$512.37	$512.35	$512.50