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	$26.33	13.871%	$29.9822	$29.98	$30.00	$30.00
Purchase #2	$92.82	8.285%	$100.5101	$100.51	$100.50	$100.50
Purchase #3	$91.65	10.127%	$100.9314	$100.93	$100.95	$100.90
Purchase #4	$40.66	10.260%	$44.8317	$44.83	$44.85	$44.80
Purchase #5	$126.83	10.456%	$140.0913	$140.09	$140.10	$140.10
Purchase #6	$109.74	4.213%	$114.3633	$114.36	$114.35	$114.40
Purchase #7	$82.95	13.339%	$94.0147	$94.01	$94.00	$94.00
7 purchase totals =	$570.98		$624.7247	$624.71	$624.75	$624.70