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	$11.86	13.819%	$13.4989	$13.50	$13.50	$13.50
Purchase #2	$127.65	8.108%	$137.9999	$138.00	$138.00	$138.00
Purchase #3	$89.74	5.720%	$94.8731	$94.87	$94.85	$94.90
Purchase #4	$123.40	7.613%	$132.7944	$132.79	$132.80	$132.80
Purchase #5	$34.52	11.665%	$38.5468	$38.55	$38.55	$38.50
Purchase #6	$230.87	5.158%	$242.7783	$242.78	$242.80	$242.80
Purchase #7	$108.72	13.788%	$123.7103	$123.71	$123.70	$123.70
7 purchase totals =	$726.76		$784.2017	$784.20	$784.20	$784.20