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	$16.35	10.271%	$18.0293	$18.03	$18.05	$18.00
Purchase #2	$148.81	6.333%	$158.2341	$158.23	$158.25	$158.20
Purchase #3	$17.59	7.192%	$18.8551	$18.86	$18.85	$18.90
Purchase #4	$27.34	5.514%	$28.8475	$28.85	$28.85	$28.80
Purchase #5	$180.38	8.310%	$195.3696	$195.37	$195.35	$195.40
Purchase #6	$155.75	7.146%	$166.8799	$166.88	$166.90	$166.90
Purchase #7	$64.34	12.182%	$72.1779	$72.18	$72.20	$72.20
7 purchase totals =	$610.56		$658.3934	$658.40	$658.45	$658.40