My experience with aftermarket wheels is that issue with incorrect combination of J and ET is that the inside of the tyre fouls the dampers. At the front this can happen when making a tight turn, at the rear when the car is heavily loaded. So it's not only a question of protruding outwards. With the correct tyre size, rim width, and ET, there shouldn't be any issues..
.I measured my 20" tires (not wheels) to fender, and the front has about 3-4mm available to go out before poking out too much, and the rear had about 7-8mm.