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	$12.30	12.847%	$13.8802	$13.88	$13.90	$13.90
Purchase #2	$187.78	12.307%	$210.8901	$210.89	$210.90	$210.90
Purchase #3	$107.26	9.287%	$117.2212	$117.22	$117.20	$117.20
Purchase #4	$143.87	8.147%	$155.5911	$155.59	$155.60	$155.60
Purchase #5	$35.42	11.426%	$39.4671	$39.47	$39.45	$39.50
Purchase #6	$147.92	13.300%	$167.5934	$167.59	$167.60	$167.60
Purchase #7	$91.93	8.768%	$99.9904	$99.99	$100.00	$100.00
7 purchase totals =	$726.48		$804.6335	$804.63	$804.65	$804.70