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	$121.34	13.845%	$138.1395	$138.14	$138.15	$138.10
Purchase #2	$126.82	10.621%	$140.2896	$140.29	$140.30	$140.30
Purchase #3	$158.38	9.520%	$173.4578	$173.46	$173.45	$173.50
Purchase #4	$50.93	6.772%	$54.379	$54.38	$54.40	$54.40
Purchase #5	$209.32	4.853%	$219.4783	$219.48	$219.50	$219.50
Purchase #6	$40.68	3.564%	$42.1298	$42.13	$42.15	$42.10
Purchase #7	$88.43	3.552%	$91.571	$91.57	$91.55	$91.60
7 purchase totals =	$795.90		$859.4450	$859.45	$859.50	$859.50