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	$19.53	12.107%	$21.8945	$21.89	$21.90	$21.90
Purchase #2	$146.94	10.111%	$161.7971	$161.80	$161.80	$161.80
Purchase #3	$50.32	11.262%	$55.987	$55.99	$56.00	$56.00
Purchase #4	$41.90	11.777%	$46.8346	$46.83	$46.85	$46.80
Purchase #5	$9.73	12.808%	$10.9762	$10.98	$11.00	$11.00
Purchase #6	$104.60	6.337%	$111.2285	$111.23	$111.25	$111.20
Purchase #7	$163.76	11.788%	$183.064	$183.06	$183.05	$183.10
7 purchase totals =	$536.78		$591.7819	$591.78	$591.85	$591.80