Depends on the offset of the wheels, for example I have found on a 40mm drop with no rubbing on 195/50/15 tires, ET28 up front and ET18-20 at the back is fine.
If your wheels are ET38 then that means 10 up front, 20 at the back
The other thing to watch out for is that you get the correct type of spacer for your car based on the thickness and which axle you fit them on. VWs are hub-centric which means the alloy wheel is centred on the centre of the hub, so if you get non-hub centric spacers you must make sure there is enough lip sticking out past them for the wheel to centre on. 10mm is as thick as you can go at the back for non-hubcentric spacers, and up front its prob around 5-8mm, but you'll have to measure to be sure. The other issue is that if you go for hub-centric spacers, you need to make sure there will be enough room inside the spacer for the hub to fit inside! I got 16mm hub-centric spacers which fitted fine on the front and back, but 10mm ones wouldn't fit flush at the front or rear
One last thing, if you do go for spacers you'll need to make sure your bolts are long enough, I found the BMW wheel bolts I was using are fine with 10mm spacers, but they were too short for 16mm ones.
