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	$38.81	8.559%	$42.1317	$42.13	$42.15	$42.10
Purchase #2	$242.28	10.367%	$267.3972	$267.40	$267.40	$267.40
Purchase #3	$231.88	4.625%	$242.6045	$242.60	$242.60	$242.60
Purchase #4	$155.42	8.472%	$168.5872	$168.59	$168.60	$168.60
Purchase #5	$31.45	11.794%	$35.1592	$35.16	$35.15	$35.20
Purchase #6	$213.44	3.583%	$221.0876	$221.09	$221.10	$221.10
Purchase #7	$5.72	10.165%	$6.3014	$6.30	$6.30	$6.30
7 purchase totals =	$919.00		$983.2688	$983.27	$983.30	$983.30