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	$18.65	13.609%	$21.1881	$21.19	$21.20	$21.20
Purchase #2	$137.66	13.695%	$156.5125	$156.51	$156.50	$156.50
Purchase #3	$75.32	6.383%	$80.1277	$80.13	$80.15	$80.10
Purchase #4	$60.53	10.469%	$66.8669	$66.87	$66.85	$66.90
Purchase #5	$66.88	9.838%	$73.4597	$73.46	$73.45	$73.50
Purchase #6	$83.39	4.514%	$87.1542	$87.15	$87.15	$87.20
Purchase #7	$103.75	7.807%	$111.8498	$111.85	$111.85	$111.80
7 purchase totals =	$546.18		$597.1589	$597.16	$597.15	$597.20