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	$36.61	10.308%	$40.3838	$40.38	$40.40	$40.40
Purchase #2	$159.39	12.102%	$178.6794	$178.68	$178.70	$178.70
Purchase #3	$37.95	3.607%	$39.3189	$39.32	$39.30	$39.30
Purchase #4	$145.33	8.307%	$157.4026	$157.40	$157.40	$157.40
Purchase #5	$26.86	9.147%	$29.3169	$29.32	$29.30	$29.30
Purchase #6	$196.46	7.151%	$210.5089	$210.51	$210.50	$210.50
Purchase #7	$9.57	8.540%	$10.3873	$10.39	$10.40	$10.40
7 purchase totals =	$612.17		$665.9978	$666.00	$666.00	$666.00