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	$16.80	10.739%	$18.6042	$18.60	$18.60	$18.60
Purchase #2	$141.75	6.207%	$150.5484	$150.55	$150.55	$150.50
Purchase #3	$139.29	3.252%	$143.8197	$143.82	$143.80	$143.80
Purchase #4	$91.77	12.369%	$103.121	$103.12	$103.10	$103.10
Purchase #5	$98.57	11.346%	$109.7538	$109.75	$109.75	$109.80
Purchase #6	$102.81	4.704%	$107.6462	$107.65	$107.65	$107.60
Purchase #7	$236.29	8.562%	$256.5211	$256.52	$256.50	$256.50
7 purchase totals =	$827.28		$890.0144	$890.01	$889.95	$889.90