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	$19.60	9.484%	$21.4589	$21.46	$21.45	$21.50
Purchase #2	$230.43	13.365%	$261.227	$261.23	$261.25	$261.20
Purchase #3	$91.81	9.486%	$100.5191	$100.52	$100.50	$100.50
Purchase #4	$123.63	5.126%	$129.9673	$129.97	$129.95	$130.00
Purchase #5	$71.79	3.837%	$74.5446	$74.54	$74.55	$74.50
Purchase #6	$130.42	4.391%	$136.1467	$136.15	$136.15	$136.10
Purchase #7	$111.64	4.338%	$116.4829	$116.48	$116.50	$116.50
7 purchase totals =	$779.32		$840.3465	$840.35	$840.35	$840.30