I've always rolled with this theory on my MTB: I like a smoother tire in the back. Right now I have a slick on the back and sort-of-knobby-but-not-really on the front for commuting.
Why? So I can have a smoother ride from the tire I am sitting on, but get traction on the front. Since I am sitting on back tire, I get all the traction that I need.
Before this, I rode mostly in the dry hills of Southern CA. There I had more aggressive tires than I do now, but I still had a smoother tire on the back. And I did pretty well.
Now, as I prepare for winter here in the Twin Cities, MN, I am thinking about my studded snows. I think I want to ride on a "street" rear with an off road front. So, less aggressive and less studs in the back, more and more in the front.
My butt is almost always in the saddle. I ride a fixed gear with a front brake.