Do you ski switch a lot? Do you want to have the same control switch as you have while skiing forwards? Then mount them true center. If you don't want to sacrifice soft snow performance, mount them at -2. The only real disadvantage of true center that I notice is when skiing powder. Depending on the ski, the tips tend to sink, and the tails tend to float, forcing a person to lean back more, which decreases control.
I mount all of my park/all mountain skis true center. I had a pair of 06/07 Chronic Blends mounted at -2, and then remounted them at true center. For the way I ski, it was a huge improvement, and I liked how the skis behaved more at true center.