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	$79.70	13.495%	$90.4555	$90.46	$90.45	$90.50
Purchase #2	$15.60	9.254%	$17.0436	$17.04	$17.05	$17.00
Purchase #3	$93.90	6.285%	$99.8016	$99.80	$99.80	$99.80
Purchase #4	$118.54	7.785%	$127.7683	$127.77	$127.75	$127.80
Purchase #5	$131.86	4.531%	$137.8346	$137.83	$137.85	$137.80
Purchase #6	$101.95	11.798%	$113.9781	$113.98	$114.00	$114.00
Purchase #7	$47.87	9.110%	$52.231	$52.23	$52.25	$52.20
7 purchase totals =	$589.42		$639.1127	$639.11	$639.15	$639.10