Appropriate Wheel Offset for 18" Wheels?

Spacers... where to start. The topic of spacers has come up multiple times on every auto forum I've been a part of and it is a very dividing one. My thoughts on them, run them if you have to and don't if you don't. I have them on the rear of my valiant for now because I can't justify spending the money on a set of wheels for a rear end I'm not going to keep.