In Western Australia you do a lot of beach work and also sharp, rocky terrain in the north. We air down for both, rocks approx 25psi. With sand it is a bit dependent on the make and style of tyre you are running, they have different deflation characteristics.
With sand you are trying to lenghen the tread footprint, with one of your tyres at normal pressure slide a ruler under the front and rear at a right angles then mark on the pavement the contact lengh. Now reduce the pressure and do the same again, you will soon see once you get under 18psi you get huge improvement in footprint. We normally drop to 20psi then if you get in trouble you can still drop further to get out. 12psi is the lowest I would go to without beadlocks and you would pump up as soon as you hit firm ground because a lot of sidewall is on the ground.