I was confortable running down to 15 psi but I was told a few different things. I was told once not to go below 15 psi without bead lockers. I was also told forget that, you can run down to 5-7 pounds and should still be fine w/33 12.5x15x8 duratracs.
So, its hard to say. Depends a lot on what your doing as well. Could prob. run at 7-10 psi in sand all day long as it is very forgiving. But rocks on the other hand... crawling raises your chances of blowing a bead tremendously compared.
That being said, what do I do??? I run 'em at 30 psi on the road and 17 psi on the trails. I don't personally have a need to go lower than about 17 unless I'm climbing big sand hills. In that case I'd run 13 but bring 'em back up before to much travel.