20mm is still quite alot unless you have uber high offset wheels, I've got 10mm at the rear on ET28 wheels and they still scrub a bit over bumps with 3 passengers... Unless you mean 20mm in total i.e. 10mmm per side?
Either way the thing to remember about spacers is you need to have a hub to centre the wheel on, either make sure there is enough of the original lip sticking out to centre the wheels, or get hub-centric spacers with their own lip on. 10mm on the rear is about as wide as you can go with non-hub centric spacers, any more and theres no lip to centre the wheel!