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	$35.69	12.181%	$40.0374	$40.04	$40.05	$40.00
Purchase #2	$250.74	9.482%	$274.5152	$274.52	$274.50	$274.50
Purchase #3	$121.37	6.647%	$129.4375	$129.44	$129.45	$129.40
Purchase #4	$105.87	11.157%	$117.6819	$117.68	$117.70	$117.70
Purchase #5	$57.41	7.419%	$61.6692	$61.67	$61.65	$61.70
Purchase #6	$21.74	12.772%	$24.5166	$24.52	$24.50	$24.50
Purchase #7	$18.91	3.834%	$19.635	$19.64	$19.65	$19.60
7 purchase totals =	$611.73		$667.4928	$667.51	$667.50	$667.40