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	$59.80	11.809%	$66.8618	$66.86	$66.85	$66.90
Purchase #2	$93.97	11.585%	$104.8564	$104.86	$104.85	$104.90
Purchase #3	$158.40	8.831%	$172.3883	$172.39	$172.40	$172.40
Purchase #4	$167.94	10.539%	$185.6392	$185.64	$185.65	$185.60
Purchase #5	$12.46	8.389%	$13.5053	$13.51	$13.50	$13.50
Purchase #6	$33.72	3.546%	$34.9157	$34.92	$34.90	$34.90
Purchase #7	$54.79	11.461%	$61.0695	$61.07	$61.05	$61.10
7 purchase totals =	$581.08		$639.2362	$639.25	$639.20	$639.30