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	$7.29	13.784%	$8.2949	$8.29	$8.30	$8.30
Purchase #2	$131.42	10.694%	$145.4741	$145.47	$145.45	$145.50
Purchase #3	$31.53	8.123%	$34.0912	$34.09	$34.10	$34.10
Purchase #4	$183.42	7.848%	$197.8148	$197.81	$197.80	$197.80
Purchase #5	$97.94	13.482%	$111.1443	$111.14	$111.15	$111.10
Purchase #6	$18.35	4.811%	$19.2328	$19.23	$19.25	$19.20
Purchase #7	$113.55	8.482%	$123.1813	$123.18	$123.20	$123.20
7 purchase totals =	$583.50		$639.2334	$639.21	$639.25	$639.20