I use about 5/8" in the front and 1" on the rear. Depending on tire width is the only difference I feel. With 225's on the car gets pulled all over by the ruts in the road. Usually at low speed. With my 215's it's fine. I threw them on for the winter.
I had 15mm. H&R's on the back only and my tires whent bald in like 4 months, not worth it to me. If you are doing some racing like auto-x then I could see them getting used but just for asthetics.....naah I would pass. if you want I will sell you my old ones...cheep. PM me if your interested.