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	$35.54	11.389%	$39.5877	$39.59	$39.60	$39.60
Purchase #2	$55.45	12.852%	$62.5764	$62.58	$62.60	$62.60
Purchase #3	$225.46	13.792%	$256.5554	$256.56	$256.55	$256.60
Purchase #4	$224.28	5.514%	$236.6468	$236.65	$236.65	$236.60
Purchase #5	$149.56	11.866%	$167.3068	$167.31	$167.30	$167.30
Purchase #6	$189.83	3.286%	$196.0678	$196.07	$196.05	$196.10
Purchase #7	$38.56	4.319%	$40.2254	$40.23	$40.25	$40.20
7 purchase totals =	$918.68		$998.9663	$998.99	$999.00	$999.00