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	$14.37	8.274%	$15.559	$15.56	$15.55	$15.60
Purchase #2	$189.54	5.689%	$200.3229	$200.32	$200.30	$200.30
Purchase #3	$146.88	10.175%	$161.825	$161.83	$161.85	$161.80
Purchase #4	$122.90	11.755%	$137.3469	$137.35	$137.35	$137.30
Purchase #5	$104.27	12.207%	$116.9982	$117.00	$117.00	$117.00
Purchase #6	$55.98	6.453%	$59.5924	$59.59	$59.60	$59.60
Purchase #7	$146.43	3.116%	$150.9928	$150.99	$151.00	$151.00
7 purchase totals =	$780.37		$842.6372	$842.64	$842.65	$842.60