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	$47.92	3.289%	$49.4961	$49.50	$49.50	$49.50
Purchase #2	$34.92	13.419%	$39.6059	$39.61	$39.60	$39.60
Purchase #3	$83.78	8.127%	$90.5888	$90.59	$90.60	$90.60
Purchase #4	$158.43	4.740%	$165.9396	$165.94	$165.95	$165.90
Purchase #5	$114.32	11.613%	$127.596	$127.60	$127.60	$127.60
Purchase #6	$93.87	7.284%	$100.7075	$100.71	$100.70	$100.70
Purchase #7	$60.93	9.360%	$66.633	$66.63	$66.65	$66.60
7 purchase totals =	$594.17		$640.5669	$640.58	$640.60	$640.50