I am running my D-Force wheels with 12mm spacers up front, and 15mm in the rear. I could definitely use 12mm all around but I find the rear wheels already too tucked in for my taste. 12mm would be worse.
Plus, I am trying to go for same track width front and back. With my setup, the front track is already about .5" wider then the rear.
Here are a couple of shots on my car: