Also depends if you are running P-metric/SL tires or LT tires (C or E load). General consensus seems to be that dropping SL tires below about 30 PSI is asking for trouble with punctures, sidewall damage, etc.
E rated can easily go down to 20 to 25 PSI or less (but you will start to lose some ground clearance which I sometimes think can be a downer).
When I ran LT265/70R17 Falkens on my previous 4Runner, I'd usually air them down to 20 to 25 PSI if I was going to be in sand or a lot of rocks. Have not aired down my Pro's stock Terra Grapplers yet which are SL load (but also haven't really taken them in anything THAT horrible).