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	$38.38	3.407%	$39.6876	$39.69	$39.70	$39.70
Purchase #2	$55.54	7.272%	$59.5789	$59.58	$59.60	$59.60
Purchase #3	$119.88	7.747%	$129.1671	$129.17	$129.15	$129.20
Purchase #4	$116.47	9.476%	$127.5067	$127.51	$127.50	$127.50
Purchase #5	$164.93	13.441%	$187.0982	$187.10	$187.10	$187.10
Purchase #6	$172.27	10.675%	$190.6598	$190.66	$190.65	$190.70
Purchase #7	$92.44	13.827%	$105.2217	$105.22	$105.20	$105.20
7 purchase totals =	$759.91		$838.9200	$838.93	$838.90	$839.00