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	$40.28	5.580%	$42.5276	$42.53	$42.55	$42.50
Purchase #2	$105.85	9.534%	$115.9417	$115.94	$115.95	$115.90
Purchase #3	$118.65	11.749%	$132.5902	$132.59	$132.60	$132.60
Purchase #4	$14.78	7.823%	$15.9362	$15.94	$15.95	$15.90
Purchase #5	$210.79	5.811%	$223.039	$223.04	$223.05	$223.00
Purchase #6	$49.77	11.274%	$55.3811	$55.38	$55.40	$55.40
Purchase #7	$54.54	4.324%	$56.8983	$56.90	$56.90	$56.90
7 purchase totals =	$594.66		$642.3141	$642.32	$642.40	$642.20