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	$35.35	12.790%	$39.8713	$39.87	$39.85	$39.90
Purchase #2	$10.28	10.561%	$11.3657	$11.37	$11.35	$11.40
Purchase #3	$165.31	5.530%	$174.4516	$174.45	$174.45	$174.50
Purchase #4	$51.79	8.243%	$56.059	$56.06	$56.05	$56.10
Purchase #5	$243.50	12.360%	$273.5966	$273.60	$273.60	$273.60
Purchase #6	$57.40	3.612%	$59.4733	$59.47	$59.45	$59.50
Purchase #7	$40.74	8.321%	$44.13	$44.13	$44.15	$44.10
7 purchase totals =	$604.37		$658.9475	$658.95	$658.90	$659.10