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	$6.84	9.732%	$7.5057	$7.51	$7.50	$7.50
Purchase #2	$74.61	12.210%	$83.7199	$83.72	$83.70	$83.70
Purchase #3	$107.99	3.527%	$111.7988	$111.80	$111.80	$111.80
Purchase #4	$111.93	8.829%	$121.8123	$121.81	$121.80	$121.80
Purchase #5	$37.76	12.448%	$42.4604	$42.46	$42.45	$42.50
Purchase #6	$126.57	11.409%	$141.0104	$141.01	$141.00	$141.00
Purchase #7	$83.80	6.352%	$89.123	$89.12	$89.10	$89.10
7 purchase totals =	$549.50		$597.4305	$597.43	$597.35	$597.40