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	$38.46	3.731%	$39.8949	$39.89	$39.90	$39.90
Purchase #2	$156.87	6.729%	$167.4258	$167.43	$167.45	$167.40
Purchase #3	$154.83	5.584%	$163.4757	$163.48	$163.50	$163.50
Purchase #4	$69.61	11.680%	$77.7404	$77.74	$77.75	$77.70
Purchase #5	$103.75	13.337%	$117.5871	$117.59	$117.60	$117.60
Purchase #6	$48.81	12.672%	$54.9952	$55.00	$55.00	$55.00
Purchase #7	$173.61	13.256%	$196.6237	$196.62	$196.60	$196.60
7 purchase totals =	$745.94		$817.7428	$817.75	$817.80	$817.70