I've always run my tires either both the same or the front bigger. Doesn't matter whether it's XC, AM, or whatever. It just makes more sense to me since the front tire does most of the work. When you are going downhill it takes all the hits and does most of the braking. Also, I like fatter tires and I can't always fit them on the rear anyway - my hardtail 29er runs a 2.1 in the rear and a 2.3 on the front.