it depends on the offset of the alloy in my opinion and what the max offset you can fit on the car. if your rims are et 45 like my 19's they sit in the arch way too deep and need taken out a bit. i used a 10mm space because they remommend an offet of no more than 35 on a leon.
but i would say going to big on the spacers will cause the wheel to foul the inner wing etc, and buying spacers that big you need to get good quality hubcentric ones (sp) for piece of mind.