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	$240.50	13.131%	$272.0801	$272.08	$272.10	$272.10
Purchase #2	$195.95	4.584%	$204.9323	$204.93	$204.95	$204.90
Purchase #3	$10.50	4.177%	$10.9386	$10.94	$10.95	$10.90
Purchase #4	$39.81	5.380%	$41.9518	$41.95	$41.95	$42.00
Purchase #5	$42.43	6.734%	$45.2872	$45.29	$45.30	$45.30
Purchase #6	$40.44	11.506%	$45.093	$45.09	$45.10	$45.10
Purchase #7	$16.58	13.338%	$18.7914	$18.79	$18.80	$18.80
7 purchase totals =	$586.21		$639.0744	$639.07	$639.15	$639.10