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	$46.49	7.554%	$50.0019	$50.00	$50.00	$50.00
Purchase #2	$89.76	12.162%	$100.6766	$100.68	$100.70	$100.70
Purchase #3	$22.82	4.110%	$23.7579	$23.76	$23.75	$23.80
Purchase #4	$228.49	5.326%	$240.6594	$240.66	$240.65	$240.70
Purchase #5	$78.72	8.593%	$85.4844	$85.48	$85.50	$85.50
Purchase #6	$62.84	5.314%	$66.1793	$66.18	$66.20	$66.20
Purchase #7	$69.70	3.501%	$72.1402	$72.14	$72.15	$72.10
7 purchase totals =	$598.82		$638.8997	$638.90	$638.95	$639.00