You can run 15mm or 20mm spacers on the rear. Width depends upon the wheels you are running, whether the car is lowered, and whether your car is a pre or post facelift.
The pre-facelift model has a lip on the rear wheel arch, the post facelift doesn't. I think they removed it on the post facelift to allow fitment of 16 inch and 195/45 tyres on the Cupra 16v models.
On my 1994 pre-facelift, lowered 40mm on Bilstein Sprintline suspension, with 15 inch standard Seat wheels and 185/55 R15 tyres I get some rubbing if I've got a lot of weight on the back, but they are fine if the car has just me in it (most of the time).
Spacing the front - people tend not to like spacing the front as much as the front wheels obviously have to steer and carry the power. It also doesn't make sense to space out the front to make it wider than the rear.
