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	$24.37	13.668%	$27.7009	$27.70	$27.70	$27.70
Purchase #2	$112.53	4.811%	$117.9438	$117.94	$117.95	$117.90
Purchase #3	$247.80	13.232%	$280.5889	$280.59	$280.60	$280.60
Purchase #4	$248.41	7.578%	$267.2345	$267.23	$267.25	$267.20
Purchase #5	$40.27	12.208%	$45.1862	$45.19	$45.20	$45.20
Purchase #6	$60.90	11.533%	$67.9236	$67.92	$67.90	$67.90
Purchase #7	$86.32	4.226%	$89.9679	$89.97	$89.95	$90.00
7 purchase totals =	$820.60		$896.5458	$896.54	$896.55	$896.50