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	$37.45	6.612%	$39.9262	$39.93	$39.95	$39.90
Purchase #2	$39.64	8.581%	$43.0415	$43.04	$43.05	$43.00
Purchase #3	$78.84	10.857%	$87.3997	$87.40	$87.40	$87.40
Purchase #4	$208.73	6.136%	$221.5377	$221.54	$221.55	$221.50
Purchase #5	$51.92	4.818%	$54.4215	$54.42	$54.40	$54.40
Purchase #6	$133.28	4.757%	$139.6201	$139.62	$139.60	$139.60
Purchase #7	$7.64	11.815%	$8.5427	$8.54	$8.55	$8.50
7 purchase totals =	$557.50		$594.4894	$594.49	$594.50	$594.30