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	$18.86	13.739%	$21.4512	$21.45	$21.45	$21.50
Purchase #2	$248.73	12.665%	$280.2317	$280.23	$280.25	$280.20
Purchase #3	$248.29	10.358%	$274.0079	$274.01	$274.00	$274.00
Purchase #4	$94.93	5.840%	$100.4739	$100.47	$100.45	$100.50
Purchase #5	$87.63	13.487%	$99.4487	$99.45	$99.45	$99.40
Purchase #6	$111.35	8.455%	$120.7646	$120.76	$120.75	$120.80
Purchase #7	$85.56	9.116%	$93.3596	$93.36	$93.35	$93.40
7 purchase totals =	$895.35		$989.7376	$989.73	$989.70	$989.80