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	$6.57	5.230%	$6.9136	$6.91	$6.90	$6.90
Purchase #2	$208.54	12.358%	$234.3114	$234.31	$234.30	$234.30
Purchase #3	$36.34	5.709%	$38.4147	$38.41	$38.40	$38.40
Purchase #4	$201.51	10.383%	$222.4328	$222.43	$222.45	$222.40
Purchase #5	$131.67	11.240%	$146.4697	$146.47	$146.45	$146.50
Purchase #6	$155.26	12.870%	$175.242	$175.24	$175.25	$175.20
Purchase #7	$58.89	13.647%	$66.9267	$66.93	$66.95	$66.90
7 purchase totals =	$798.78		$890.7109	$890.70	$890.70	$890.60