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	$19.79	6.224%	$21.0217	$21.02	$21.00	$21.00
Purchase #2	$82.99	8.700%	$90.2101	$90.21	$90.20	$90.20
Purchase #3	$20.98	11.388%	$23.3692	$23.37	$23.35	$23.40
Purchase #4	$160.51	6.202%	$170.4648	$170.46	$170.45	$170.50
Purchase #5	$85.82	13.778%	$97.6443	$97.64	$97.65	$97.60
Purchase #6	$105.92	12.681%	$119.3517	$119.35	$119.35	$119.40
Purchase #7	$76.86	11.605%	$85.7796	$85.78	$85.80	$85.80
7 purchase totals =	$552.87		$607.8414	$607.83	$607.80	$607.90