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	$34.34	13.683%	$39.0387	$39.04	$39.05	$39.00
Purchase #2	$145.58	3.666%	$150.917	$150.92	$150.90	$150.90
Purchase #3	$181.68	10.395%	$200.5656	$200.57	$200.55	$200.60
Purchase #4	$75.91	3.422%	$78.5076	$78.51	$78.50	$78.50
Purchase #5	$236.91	8.624%	$257.3411	$257.34	$257.35	$257.30
Purchase #6	$136.34	10.204%	$150.2521	$150.25	$150.25	$150.30
Purchase #7	$24.66	10.513%	$27.2525	$27.25	$27.25	$27.30
7 purchase totals =	$835.42		$903.8746	$903.88	$903.85	$903.90