Different tires can require different pressures. 29 is a very good all around pressure, Grassroots magazine (NASA magazine) did some extensive testing on pressures, as you go up the handling will get more precise but ride quality will suffer as will tire wear, going below 29 is almost never a good idea except for drag racing, and then only the back like Joe says. You can adjust oversteer and understeer on the car by changing the balance front to rear, increasing front tire pressure tends to reduce understeer, increasing rear pressure will reduce grip and allow the rear to break loose sooner. I run my factory tires at 29, but my Bridgestones at 31, there are reasons to run it a different pressures. I believe the Auto X guys run them up to 35 and 40 for racing.
I watched Rhys Millen set the car up before the race at Road Atlanta, he spent half a day getting the tire pressures dialed in. I remember thinking it was very odd he ended up with 25 up front and 35 in the back, but he runs 225 tires up front and 265's in the back. He adjusts the pressure for every track.
