A spacer will only reduce offset numbers and increase the distance between the wheel/tire combo and the suspension (usually the shock or strut). At the same time it reduces the distance between the tire and the fender lip or fender liner. This is where most rubbing occurs. Unless you can flare the fenders and beef up the hub assembly (more stress will be placed on the wheel bearings) most spacers afford little benefit.
Sometimes..... it really depends on the thickness of the spacers and the wheel flange thickness. You want the lug nut to catch a lot of threads not just a few. You need at least 1/2 to 3/4 of an inch (beyond the nuts seat on the wheel) for proper threading and torquing. The more the better but sometimes the length is also dictated by the style of the lug nut as well since a closed end nut can only be threaded on so far before it bottoms out. Naturally there's not much of a problem with an open ended lug nut
possibly down the road i may get some 17x8 with a 20 offset from discount tires.. would need to flare the fenders a lot though to get it to work. it wouldnt be any closer to the suspension, but itd be an inch farther out towards the fender. course with 235/40 tires they would be .5" shorter which would help