Appropriate Wheel Offset for 18" Wheels?

I just placed my order on wheels. I'm wondering if I should order 245/40/18's and start by there or just stick with the 255/40/18's and fiddle around. Remeasuring, I think I'm going to need some small spacers but as long as I have good thread engagement, I think I should be fine. I can't afford to have the wheel hit the frame rails due to brake lines.