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	$249.71	11.627%	$278.7438	$278.74	$278.75	$278.70
Purchase #2	$83.32	13.293%	$94.3957	$94.40	$94.40	$94.40
Purchase #3	$136.54	9.862%	$150.0056	$150.01	$150.00	$150.00
Purchase #4	$70.30	4.422%	$73.4087	$73.41	$73.40	$73.40
Purchase #5	$142.82	5.409%	$150.5451	$150.55	$150.55	$150.50
Purchase #6	$73.26	7.167%	$78.5105	$78.51	$78.50	$78.50
Purchase #7	$166.98	3.643%	$173.0631	$173.06	$173.05	$173.10
7 purchase totals =	$922.93		$998.6725	$998.68	$998.65	$998.60