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	$5.82	13.790%	$6.6226	$6.62	$6.60	$6.60
Purchase #2	$153.84	9.762%	$168.8579	$168.86	$168.85	$168.90
Purchase #3	$113.65	3.324%	$117.4277	$117.43	$117.45	$117.40
Purchase #4	$34.36	13.126%	$38.8701	$38.87	$38.85	$38.90
Purchase #5	$89.62	11.414%	$99.8492	$99.85	$99.85	$99.80
Purchase #6	$82.99	8.513%	$90.0549	$90.05	$90.05	$90.10
Purchase #7	$117.57	12.420%	$132.1722	$132.17	$132.15	$132.20
7 purchase totals =	$597.85		$653.8546	$653.85	$653.80	$653.90