Depends on the tire. My TKC 80s like to be aired up, even off pavement I rarely drop the pressure below 27/28 psi. With the Kenda 270s I would go as low as 20 psi.
If you go too low (like less than 20 +/-, depending on tire), make sure you have rim locks installed so you don't spin the tire on the rim and rip a valve stem.