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	$45.25	13.703%	$51.4506	$51.45	$51.45	$51.50
Purchase #2	$24.55	11.493%	$27.3715	$27.37	$27.35	$27.40
Purchase #3	$66.72	8.321%	$72.2718	$72.27	$72.25	$72.30
Purchase #4	$140.43	9.461%	$153.7161	$153.72	$153.70	$153.70
Purchase #5	$129.60	11.118%	$144.0089	$144.01	$144.00	$144.00
Purchase #6	$166.28	4.330%	$173.4799	$173.48	$173.50	$173.50
Purchase #7	$151.52	4.332%	$158.0838	$158.08	$158.10	$158.10
7 purchase totals =	$724.35		$780.3826	$780.38	$780.35	$780.50