I'm contemplating the same thing for my 97 Jetta. A local shop suggested 8mm to 10mm spacers. They said any larger than that and I'd have to switch over from bolts to studs and might run into clearance issues.
I should clarify....with the 10mm spacers, I would have to run a longer bolt. If I go larger than a 10mm spacer, they told me I would have to switch the car over to studs. They did not tell me I could stick with my stock bolts. Sorry if it sounded that way.