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	$122.90	11.135%	$136.5849	$136.58	$136.60	$136.60
Purchase #2	$59.72	10.262%	$65.8485	$65.85	$65.85	$65.80
Purchase #3	$48.52	8.233%	$52.5147	$52.51	$52.50	$52.50
Purchase #4	$68.44	6.180%	$72.6696	$72.67	$72.65	$72.70
Purchase #5	$36.68	11.332%	$40.8366	$40.84	$40.85	$40.80
Purchase #6	$220.74	9.845%	$242.4719	$242.47	$242.45	$242.50
Purchase #7	$233.95	6.376%	$248.8667	$248.87	$248.85	$248.90
7 purchase totals =	$790.95		$859.7929	$859.79	$859.75	$859.80