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	$6.65	12.665%	$7.4922	$7.49	$7.50	$7.50
Purchase #2	$36.59	9.302%	$39.9936	$39.99	$40.00	$40.00
Purchase #3	$191.93	7.690%	$206.6894	$206.69	$206.70	$206.70
Purchase #4	$184.89	6.320%	$196.575	$196.58	$196.60	$196.60
Purchase #5	$111.56	10.432%	$123.1979	$123.20	$123.20	$123.20
Purchase #6	$220.90	13.430%	$250.5669	$250.57	$250.55	$250.60
Purchase #7	$162.33	10.808%	$179.8746	$179.87	$179.85	$179.90
7 purchase totals =	$914.85		$1,004.3896	$1,004.39	$1,004.40	$1,004.50