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	$93.33	6.168%	$99.0866	$99.09	$99.10	$99.10
Purchase #2	$59.62	13.211%	$67.4964	$67.50	$67.50	$67.50
Purchase #3	$87.91	3.495%	$90.9825	$90.98	$91.00	$91.00
Purchase #4	$43.61	8.804%	$47.4494	$47.45	$47.45	$47.40
Purchase #5	$99.66	8.488%	$108.1191	$108.12	$108.10	$108.10
Purchase #6	$126.71	7.211%	$135.8471	$135.85	$135.85	$135.80
Purchase #7	$108.71	8.458%	$117.9047	$117.90	$117.90	$117.90
7 purchase totals =	$619.55		$666.8858	$666.89	$666.90	$666.80