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	$37.37	12.587%	$42.0738	$42.07	$42.05	$42.10
Purchase #2	$191.56	12.500%	$215.505	$215.51	$215.50	$215.50
Purchase #3	$190.37	7.855%	$205.3236	$205.32	$205.30	$205.30
Purchase #4	$37.26	6.325%	$39.6167	$39.62	$39.60	$39.60
Purchase #5	$34.76	12.162%	$38.9875	$38.99	$39.00	$39.00
Purchase #6	$13.48	10.858%	$14.9437	$14.94	$14.95	$14.90
Purchase #7	$18.92	10.122%	$20.8351	$20.84	$20.85	$20.80
7 purchase totals =	$523.72		$577.2854	$577.29	$577.25	$577.20