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	$35.48	10.695%	$39.2746	$39.27	$39.25	$39.30
Purchase #2	$71.70	8.695%	$77.9343	$77.93	$77.95	$77.90
Purchase #3	$244.64	4.829%	$256.4537	$256.45	$256.45	$256.50
Purchase #4	$108.78	13.823%	$123.8167	$123.82	$123.80	$123.80
Purchase #5	$187.54	11.797%	$209.6641	$209.66	$209.65	$209.70
Purchase #6	$192.55	6.290%	$204.6614	$204.66	$204.65	$204.70
Purchase #7	$72.55	7.488%	$77.9825	$77.98	$78.00	$78.00
7 purchase totals =	$913.24		$989.7873	$989.77	$989.75	$989.90