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	$81.83	10.583%	$90.4901	$90.49	$90.50	$90.50
Purchase #2	$212.94	8.128%	$230.2478	$230.25	$230.25	$230.20
Purchase #3	$25.98	9.547%	$28.4603	$28.46	$28.45	$28.50
Purchase #4	$83.77	3.299%	$86.5336	$86.53	$86.55	$86.50
Purchase #5	$62.25	8.562%	$67.5798	$67.58	$67.60	$67.60
Purchase #6	$63.83	3.194%	$65.8687	$65.87	$65.85	$65.90
Purchase #7	$28.99	3.715%	$30.067	$30.07	$30.05	$30.10
7 purchase totals =	$559.59		$599.2473	$599.25	$599.25	$599.30