Jerebus, you don't need to narrow the axle, but Imo if you're running a 315 or 335 tire on an eleven or twelve inch rim the offset required to do so just doesn't look right.
But, it is your car so the desicion is ultimately your own.
I went with the 335/30/18 on an 18x12 Budnik Fontana with 6" of backspace.
You can also go to the Lateral-G homepage to see wheels in similar sizes and backspace installed on cars.