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	$27.78	13.577%	$31.5517	$31.55	$31.55	$31.60
Purchase #2	$138.72	13.491%	$157.4347	$157.43	$157.45	$157.40
Purchase #3	$39.25	4.119%	$40.8667	$40.87	$40.85	$40.90
Purchase #4	$86.65	12.376%	$97.3738	$97.37	$97.35	$97.40
Purchase #5	$45.55	12.557%	$51.2697	$51.27	$51.25	$51.30
Purchase #6	$102.53	13.651%	$116.5264	$116.53	$116.55	$116.50
Purchase #7	$9.88	10.280%	$10.8957	$10.90	$10.90	$10.90
7 purchase totals =	$450.36		$505.9187	$505.92	$505.90	$506.00