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	$34.94	12.605%	$39.3442	$39.34	$39.35	$39.30
Purchase #2	$90.40	10.809%	$100.1713	$100.17	$100.15	$100.20
Purchase #3	$186.87	11.812%	$208.9431	$208.94	$208.95	$208.90
Purchase #4	$218.54	3.342%	$225.8436	$225.84	$225.85	$225.80
Purchase #5	$46.26	9.872%	$50.8268	$50.83	$50.85	$50.80
Purchase #6	$155.55	11.483%	$173.4118	$173.41	$173.40	$173.40
Purchase #7	$104.29	13.121%	$117.9739	$117.97	$117.95	$118.00
7 purchase totals =	$836.85		$916.5147	$916.50	$916.50	$916.40