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	$88.84	11.860%	$99.3764	$99.38	$99.40	$99.40
Purchase #2	$100.92	12.669%	$113.7056	$113.71	$113.70	$113.70
Purchase #3	$35.30	10.314%	$38.9408	$38.94	$38.95	$38.90
Purchase #4	$132.40	3.268%	$136.7268	$136.73	$136.75	$136.70
Purchase #5	$85.85	11.659%	$95.8593	$95.86	$95.85	$95.90
Purchase #6	$143.89	3.400%	$148.7823	$148.78	$148.80	$148.80
Purchase #7	$25.82	3.597%	$26.7487	$26.75	$26.75	$26.70
7 purchase totals =	$613.02		$660.1399	$660.15	$660.20	$660.10