I think the 17s have a different offset to the 18s. I know someone with 17s and 20mm spacers on the back though. I don't think he has anything on the front. I would have thought that 12mm ones would be fine though.
I've had the springs and spacers for quite a while waiting to go on - just needed the car lol. Might as well do it when the car is new and the bolts all come undone easily and the dampers aren't so easily upset by a different ride height.