a 2.3 is the smallest i run. lately i find myself running 2.5's and 2.7's sometimes. i've become a real fan of the big fat meats. i could understand wanting to mix it up and run a bigger tire in the front than the back, but honestly its not a practice that has appealed to my riding. but i'm on a hardtail, so i appreciate the extra give back there.

though really, for my riding today (fairly smooth trails on frozen mud) i could have had a thinner tire and been a bit faster. maybe for just such an occasion its time for me to pick up some 1.9's.

i guess, as kind of a not very good answer to your question; buy lots of tires. buy a tire that you think would be good for any of the conditions you ride in, and swap around as you see fit. if you think a bigger tire up front will work better, go for it.