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	$34.71	13.556%	$39.4153	$39.42	$39.40	$39.40
Purchase #2	$106.72	5.459%	$112.5458	$112.55	$112.55	$112.50
Purchase #3	$74.93	11.460%	$83.517	$83.52	$83.50	$83.50
Purchase #4	$126.42	11.780%	$141.3123	$141.31	$141.30	$141.30
Purchase #5	$165.41	8.501%	$179.4715	$179.47	$179.45	$179.50
Purchase #6	$108.98	7.477%	$117.1284	$117.13	$117.15	$117.10
Purchase #7	$129.88	5.244%	$136.6909	$136.69	$136.70	$136.70
7 purchase totals =	$747.05		$810.0812	$810.09	$810.05	$810.00