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	$36.45	9.435%	$39.8891	$39.89	$39.90	$39.90
Purchase #2	$210.48	13.767%	$239.4568	$239.46	$239.45	$239.50
Purchase #3	$36.66	7.817%	$39.5257	$39.53	$39.55	$39.50
Purchase #4	$102.98	3.468%	$106.5513	$106.55	$106.55	$106.60
Purchase #5	$126.50	8.669%	$137.4663	$137.47	$137.45	$137.50
Purchase #6	$19.87	10.455%	$21.9474	$21.95	$21.95	$21.90
Purchase #7	$35.88	3.870%	$37.2686	$37.27	$37.25	$37.30
7 purchase totals =	$568.82		$622.1052	$622.12	$622.10	$622.20