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	$12.89	4.684%	$13.4938	$13.49	$13.50	$13.50
Purchase #2	$237.83	11.144%	$264.3338	$264.33	$264.35	$264.30
Purchase #3	$199.85	13.873%	$227.5752	$227.58	$227.60	$227.60
Purchase #4	$95.75	12.158%	$107.3913	$107.39	$107.40	$107.40
Purchase #5	$135.89	5.616%	$143.5216	$143.52	$143.50	$143.50
Purchase #6	$35.85	12.390%	$40.2918	$40.29	$40.30	$40.30
Purchase #7	$39.71	11.457%	$44.2596	$44.26	$44.25	$44.30
7 purchase totals =	$757.77		$840.8671	$840.86	$840.90	$840.90