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	$34.89	8.849%	$37.9774	$37.98	$38.00	$38.00
Purchase #2	$207.79	13.725%	$236.3092	$236.31	$236.30	$236.30
Purchase #3	$54.79	9.368%	$59.9227	$59.92	$59.90	$59.90
Purchase #4	$131.78	6.462%	$140.2956	$140.30	$140.30	$140.30
Purchase #5	$26.67	10.366%	$29.4346	$29.43	$29.45	$29.40
Purchase #6	$141.96	8.795%	$154.4454	$154.45	$154.45	$154.40
Purchase #7	$18.32	9.688%	$20.0948	$20.09	$20.10	$20.10
7 purchase totals =	$616.20		$678.4797	$678.48	$678.50	$678.40