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	$17.89	12.775%	$20.1754	$20.18	$20.20	$20.20
Purchase #2	$67.92	13.160%	$76.8583	$76.86	$76.85	$76.90
Purchase #3	$106.32	11.105%	$118.1268	$118.13	$118.15	$118.10
Purchase #4	$163.68	6.397%	$174.1506	$174.15	$174.15	$174.20
Purchase #5	$53.72	12.330%	$60.3437	$60.34	$60.35	$60.30
Purchase #6	$35.96	11.679%	$40.1598	$40.16	$40.15	$40.20
Purchase #7	$84.30	6.709%	$89.9557	$89.96	$89.95	$90.00
7 purchase totals =	$529.79		$579.7703	$579.78	$579.80	$579.90