Depends on your tolerance for ride comfort vs. treadwear. Higher pressure gives you better cornering and better treadwear, but the ride quality gets downright bone-jarring on 18"s.
Personally, I run 40-42psi all the way around, but ADS makes the ride more tolerable than the stock suspension (when it's working right anyway...which is rare). On my Dad's car with Bilstein HD's 32-36psi is all you can get away with before it starts rattling your fillings out of your teeth. Overall, the Michelin PS's like higher pressures...they wear really quick once you get down into the 30psi's.
