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	$127.94	8.792%	$139.1885	$139.19	$139.20	$139.20
Purchase #2	$88.82	6.804%	$94.8633	$94.86	$94.85	$94.90
Purchase #3	$179.33	6.473%	$190.938	$190.94	$190.95	$190.90
Purchase #4	$65.75	10.821%	$72.8648	$72.86	$72.85	$72.90
Purchase #5	$95.50	3.565%	$98.9046	$98.90	$98.90	$98.90
Purchase #6	$40.48	10.678%	$44.8025	$44.80	$44.80	$44.80
Purchase #7	$16.80	8.699%	$18.2614	$18.26	$18.25	$18.30
7 purchase totals =	$614.62		$659.8231	$659.81	$659.80	$659.90