ok,
off the top of my head, the offset should be such for optimal fitment:
6" wide wheel et30-40
6.5" et25-35
7" et20-30
7.5" et20-25 (Fender rolling suggested)
8" et17-20 (fender rolling and added negative camber suggested)
You can see that as the wheels get wider, you have a smaller window for error.
In the rear, negative camber is increased as a result of lowering, in the front you will need camber plates to replace your strut mounts.
So when do you need spacers? If you have a 7" wide whee that has an offset of 40mm you will need a 10-20mm spacer to get the offset down to the recommended window.