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	$238.89	13.393%	$270.8845	$270.88	$270.90	$270.90
Purchase #2	$35.83	10.657%	$39.6484	$39.65	$39.65	$39.60
Purchase #3	$150.47	12.185%	$168.8048	$168.80	$168.80	$168.80
Purchase #4	$128.27	10.332%	$141.5229	$141.52	$141.50	$141.50
Purchase #5	$34.42	10.201%	$37.9312	$37.93	$37.95	$37.90
Purchase #6	$248.30	5.399%	$261.7057	$261.71	$261.70	$261.70
Purchase #7	$7.70	7.612%	$8.2861	$8.29	$8.30	$8.30
7 purchase totals =	$843.88		$928.7836	$928.78	$928.80	$928.70