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	$17.83	13.806%	$20.2916	$20.29	$20.30	$20.30
Purchase #2	$52.58	4.687%	$55.0444	$55.04	$55.05	$55.00
Purchase #3	$140.97	12.745%	$158.9366	$158.94	$158.95	$158.90
Purchase #4	$83.61	11.682%	$93.3773	$93.38	$93.40	$93.40
Purchase #5	$130.75	10.226%	$144.1205	$144.12	$144.10	$144.10
Purchase #6	$87.71	13.415%	$99.4763	$99.48	$99.50	$99.50
Purchase #7	$75.72	6.312%	$80.4994	$80.50	$80.50	$80.50
7 purchase totals =	$589.17		$651.7461	$651.75	$651.80	$651.70