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	$34.52	13.868%	$39.3072	$39.31	$39.30	$39.30
Purchase #2	$215.85	11.224%	$240.077	$240.08	$240.10	$240.10
Purchase #3	$107.27	10.487%	$118.5194	$118.52	$118.50	$118.50
Purchase #4	$139.35	4.733%	$145.9454	$145.95	$145.95	$145.90
Purchase #5	$185.78	5.523%	$196.0406	$196.04	$196.05	$196.00
Purchase #6	$84.31	13.835%	$95.9743	$95.97	$95.95	$96.00
Purchase #7	$114.84	12.242%	$128.8987	$128.90	$128.90	$128.90
7 purchase totals =	$881.92		$964.7626	$964.77	$964.75	$964.70