I've always gone by the terrain ahead. Rocks are different than sand, as mud is different to snow. And when you talk about airing down to wheel in snow/ice, there're a few schools of thought there as well.
Most of my off roading's on rocks, and I've run 33" DuellerMTs at 10 and FStone Desti MTs (33) (yes, I know it's metric..) at 12-15. Now, for something like slickrock, in Sedona, or Moab, I'm happy around 18-20. Still a good flat wide footprint, and the sidewalls retain their rigidity. That's a good pressure for "boony-whompin'" down dirt roads/washes, or over long washboards too. Smooths out the ride considerably, so you can spend more time, boony-whompin'. I've been almost impressed with the stock OR tires coincidentally, for what I've put them through so far, I'd expected carnage already, but no.
