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	$17.38	3.847%	$18.0486	$18.05	$18.05	$18.00
Purchase #2	$148.43	6.864%	$158.6182	$158.62	$158.60	$158.60
Purchase #3	$104.25	6.863%	$111.4047	$111.40	$111.40	$111.40
Purchase #4	$138.84	8.529%	$150.6817	$150.68	$150.70	$150.70
Purchase #5	$10.68	7.463%	$11.477	$11.48	$11.50	$11.50
Purchase #6	$99.34	3.390%	$102.7076	$102.71	$102.70	$102.70
Purchase #7	$81.55	5.804%	$86.2832	$86.28	$86.30	$86.30
7 purchase totals =	$600.47		$639.2210	$639.22	$639.25	$639.20