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	$39.59	6.439%	$42.1392	$42.14	$42.15	$42.10
Purchase #2	$111.61	13.623%	$126.8146	$126.81	$126.80	$126.80
Purchase #3	$243.59	13.216%	$275.7829	$275.78	$275.80	$275.80
Purchase #4	$91.48	9.642%	$100.3005	$100.30	$100.30	$100.30
Purchase #5	$235.59	5.575%	$248.7241	$248.72	$248.70	$248.70
Purchase #6	$10.85	9.117%	$11.8392	$11.84	$11.85	$11.80
Purchase #7	$69.32	9.527%	$75.9241	$75.92	$75.90	$75.90
7 purchase totals =	$802.03		$881.5246	$881.51	$881.50	$881.40