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	$18.78	5.284%	$19.7723	$19.77	$19.75	$19.80
Purchase #2	$19.58	7.631%	$21.0741	$21.07	$21.05	$21.10
Purchase #3	$216.46	11.422%	$241.1841	$241.18	$241.20	$241.20
Purchase #4	$67.62	6.453%	$71.9835	$71.98	$72.00	$72.00
Purchase #5	$8.58	12.339%	$9.6387	$9.64	$9.65	$9.60
Purchase #6	$198.50	8.585%	$215.5412	$215.54	$215.55	$215.50
Purchase #7	$57.43	4.268%	$59.8811	$59.88	$59.90	$59.90
7 purchase totals =	$586.95		$639.0750	$639.06	$639.10	$639.10