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	$5.83	13.189%	$6.5989	$6.60	$6.60	$6.60
Purchase #2	$167.75	10.798%	$185.8636	$185.86	$185.85	$185.90
Purchase #3	$146.57	5.859%	$155.1575	$155.16	$155.15	$155.20
Purchase #4	$103.83	7.145%	$111.2487	$111.25	$111.25	$111.20
Purchase #5	$244.51	3.417%	$252.8649	$252.86	$252.85	$252.90
Purchase #6	$95.52	11.841%	$106.8305	$106.83	$106.85	$106.80
Purchase #7	$221.62	8.814%	$241.1536	$241.15	$241.15	$241.20
7 purchase totals =	$985.63		$1,059.7177	$1,059.71	$1,059.70	$1,059.80