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	$38.33	10.832%	$42.4819	$42.48	$42.50	$42.50
Purchase #2	$234.35	11.164%	$260.5128	$260.51	$260.50	$260.50
Purchase #3	$36.52	6.809%	$39.0066	$39.01	$39.00	$39.00
Purchase #4	$45.68	7.699%	$49.1969	$49.20	$49.20	$49.20
Purchase #5	$174.89	5.521%	$184.5457	$184.55	$184.55	$184.50
Purchase #6	$38.42	11.304%	$42.763	$42.76	$42.75	$42.80
Purchase #7	$191.82	12.785%	$216.3442	$216.34	$216.35	$216.30
7 purchase totals =	$760.01		$834.8511	$834.85	$834.85	$834.80