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	$37.87	4.764%	$39.6741	$39.67	$39.65	$39.70
Purchase #2	$36.37	6.452%	$38.7166	$38.72	$38.70	$38.70
Purchase #3	$85.29	6.333%	$90.6914	$90.69	$90.70	$90.70
Purchase #4	$199.87	12.274%	$224.402	$224.40	$224.40	$224.40
Purchase #5	$14.48	11.626%	$16.1634	$16.16	$16.15	$16.20
Purchase #6	$142.87	5.600%	$150.8707	$150.87	$150.85	$150.90
Purchase #7	$185.79	13.472%	$210.8196	$210.82	$210.80	$210.80
7 purchase totals =	$702.54		$771.3378	$771.33	$771.25	$771.40