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	$38.48	6.316%	$40.9104	$40.91	$40.90	$40.90
Purchase #2	$212.81	12.559%	$239.5368	$239.54	$239.55	$239.50
Purchase #3	$67.62	10.456%	$74.6903	$74.69	$74.70	$74.70
Purchase #4	$30.36	8.758%	$33.0189	$33.02	$33.00	$33.00
Purchase #5	$95.29	10.702%	$105.4879	$105.49	$105.50	$105.50
Purchase #6	$111.68	6.397%	$118.8242	$118.82	$118.80	$118.80
Purchase #7	$66.52	8.741%	$72.3345	$72.33	$72.35	$72.30
7 purchase totals =	$622.76		$684.8030	$684.80	$684.80	$684.70