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	$21.57	11.785%	$24.112	$24.11	$24.10	$24.10
Purchase #2	$220.94	12.619%	$248.8204	$248.82	$248.80	$248.80
Purchase #3	$14.40	8.779%	$15.6642	$15.66	$15.65	$15.70
Purchase #4	$36.69	4.568%	$38.366	$38.37	$38.35	$38.40
Purchase #5	$106.69	4.541%	$111.5348	$111.53	$111.55	$111.50
Purchase #6	$222.55	8.713%	$241.9408	$241.94	$241.95	$241.90
Purchase #7	$171.53	10.781%	$190.0226	$190.02	$190.00	$190.00
7 purchase totals =	$794.37		$870.4608	$870.45	$870.40	$870.40