I run spacers all the time with no problems. The best spacers I have found are from ECS Tuning:
ECS News Porsche 986 Boxster/S ECS Wheel Spacers
They are hub-centric, and include the appropriate length lugs in each kit. One unique feature is that they also include longer screws into the brake disc top hat, so that you can screw these spacers onto the top hat if you want, and they will stay in place whenever you take the wheels off. I personally do not use this feature though, as I change out spacers to optimize the track for each wheel, and I use a lot of different wheels.
As the previous poster noted, you probaly want the larger spacer on the front to maximize track width at the front. This will help reduce weight transfer at the front, and help reduce the gross understeer these cars have when stock.