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	$35.34	10.391%	$39.0122	$39.01	$39.00	$39.00
Purchase #2	$123.82	12.106%	$138.8096	$138.81	$138.80	$138.80
Purchase #3	$131.92	10.367%	$145.5961	$145.60	$145.60	$145.60
Purchase #4	$180.82	8.444%	$196.0884	$196.09	$196.10	$196.10
Purchase #5	$44.65	11.873%	$49.9513	$49.95	$49.95	$50.00
Purchase #6	$57.65	13.275%	$65.303	$65.30	$65.30	$65.30
Purchase #7	$29.76	4.393%	$31.0674	$31.07	$31.05	$31.10
7 purchase totals =	$603.96		$665.8280	$665.83	$665.80	$665.90