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	$249.45	11.654%	$278.5209	$278.52	$278.50	$278.50
Purchase #2	$33.89	13.341%	$38.4113	$38.41	$38.40	$38.40
Purchase #3	$112.99	13.725%	$128.4979	$128.50	$128.50	$128.50
Purchase #4	$22.69	3.512%	$23.4869	$23.49	$23.50	$23.50
Purchase #5	$100.60	3.420%	$104.0405	$104.04	$104.05	$104.00
Purchase #6	$230.51	8.177%	$249.3588	$249.36	$249.35	$249.40
Purchase #7	$16.76	4.773%	$17.56	$17.56	$17.55	$17.60
7 purchase totals =	$766.89		$839.8763	$839.88	$839.85	$839.90