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	$18.46	8.739%	$20.0732	$20.07	$20.05	$20.10
Purchase #2	$63.82	4.803%	$66.8853	$66.89	$66.90	$66.90
Purchase #3	$135.83	5.570%	$143.3957	$143.40	$143.40	$143.40
Purchase #4	$34.49	11.350%	$38.4046	$38.40	$38.40	$38.40
Purchase #5	$146.94	5.804%	$155.4684	$155.47	$155.45	$155.50
Purchase #6	$30.46	5.635%	$32.1764	$32.18	$32.20	$32.20
Purchase #7	$185.83	13.141%	$210.2499	$210.25	$210.25	$210.20
7 purchase totals =	$615.83		$666.6535	$666.66	$666.65	$666.70