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	$27.90	12.573%	$31.4079	$31.41	$31.40	$31.40
Purchase #2	$95.47	12.681%	$107.5766	$107.58	$107.60	$107.60
Purchase #3	$67.43	7.164%	$72.2607	$72.26	$72.25	$72.30
Purchase #4	$48.63	9.412%	$53.2071	$53.21	$53.20	$53.20
Purchase #5	$30.45	4.452%	$31.8056	$31.81	$31.80	$31.80
Purchase #6	$230.40	9.106%	$251.3802	$251.38	$251.40	$251.40
Purchase #7	$107.43	10.306%	$118.5017	$118.50	$118.50	$118.50
7 purchase totals =	$607.71		$666.1398	$666.15	$666.15	$666.20