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	$15.85	13.509%	$17.9912	$17.99	$18.00	$18.00
Purchase #2	$37.95	10.751%	$42.03	$42.03	$42.05	$42.00
Purchase #3	$77.90	10.858%	$86.3584	$86.36	$86.35	$86.40
Purchase #4	$168.65	11.800%	$188.5507	$188.55	$188.55	$188.60
Purchase #5	$141.56	8.520%	$153.6209	$153.62	$153.60	$153.60
Purchase #6	$141.76	7.772%	$152.7776	$152.78	$152.80	$152.80
Purchase #7	$158.86	6.755%	$169.591	$169.59	$169.60	$169.60
7 purchase totals =	$742.53		$810.9198	$810.92	$810.95	$811.00