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	$8.44	5.312%	$8.8883	$8.89	$8.90	$8.90
Purchase #2	$90.80	5.414%	$95.7159	$95.72	$95.70	$95.70
Purchase #3	$38.57	9.605%	$42.2746	$42.27	$42.25	$42.30
Purchase #4	$136.26	6.187%	$144.6904	$144.69	$144.70	$144.70
Purchase #5	$209.44	8.584%	$227.4183	$227.42	$227.40	$227.40
Purchase #6	$86.58	5.176%	$91.0614	$91.06	$91.05	$91.10
Purchase #7	$206.47	11.311%	$229.8238	$229.82	$229.80	$229.80
7 purchase totals =	$776.56		$839.8727	$839.87	$839.80	$839.90