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	$7.91	6.399%	$8.4162	$8.42	$8.40	$8.40
Purchase #2	$144.88	13.605%	$164.5909	$164.59	$164.60	$164.60
Purchase #3	$15.77	13.341%	$17.8739	$17.87	$17.85	$17.90
Purchase #4	$25.77	8.863%	$28.054	$28.05	$28.05	$28.10
Purchase #5	$60.34	11.417%	$67.229	$67.23	$67.25	$67.20
Purchase #6	$139.82	8.359%	$151.5076	$151.51	$151.50	$151.50
Purchase #7	$250.67	12.851%	$282.8836	$282.88	$282.90	$282.90
7 purchase totals =	$645.16		$720.5552	$720.55	$720.55	$720.60