Originally Posted by SpiderMike
On my fully rigid SS, I run a 26x2.5 up front, and a 26x2.2 in the back. The treads are different as stated above for getting the most out of that wheel. Another reason I run a bigger tire up front is to help soak up the bumps. When I first built up the bike I ran a 2.2 upfront. Since putting a 2.5 up front, I've noticed some difference in the steering characteristics. But I'm racking that up to a tire with better side grip.
A good example of what cyccomute is saying, would be the Panaracer Smoke and Dart tires. The rear (Smoke) has a "paddle" like design for laying down grip for acceleration/climbing. While the front (Dart) has an arrow like pattern for more lateral grip.
http://www.panaracer.com/eng/products/mtb/xc.html
I was going to use that as an example but decided not to! When the Smoke came out...long ago...I ran them on both ends. Big mistake! The Smoke would hold on to the ground in a turn or on rocks to the very end and then suddenly break loose, usually leaving me on the ground. It was a very scary front tire. Put a Dart on and the bike goes where you point it. They are still a great tire and they wear like iron.