I don't know about sand, but in rocks airing down will help your traction. It will also make you more suseptible to popping a bead. I usually air down to 15 to 18lbs depending on my load and have never had a flat. I know people who go down to 9 or 10 lbs and haven't had a problem. 15 to 20lbs is also easier on your suspension and, like another poster said, your kidneys. On the other hand, in the sand it may not matter, but in the rocks airing down will lower your jeep and cause you to hang up on "stuff". Carry a jack and a spare and go for it.

 

Airing down on the trail is the only way to go. If you don't want to at first you can always do it once you can't get past an obstical that everyone else crawled right over. 10-15 psi with 33 on stock wheels will be fine. As far as 35psi on the street is probably to high. I'm running 28psi with 33x12.5 on 16x10. If you take some tire chalk and draw a line across your tire you can adjust your air pressure until the chalk wears of evenly when driving in a straight line. That will give your proper air presure for the street.