My fronts run at 80psi and each tire in the rear run at 65psi (door jam recommended tire pressures) during highway driving.
My F350 tyre pressures are dictated by the load on the wheels, not by what is on a sticker.
Like the OP I drop the pressures when I'm likely to be on poor roads for a long distance, especially corrugations. Even a few psi out of the front tyres can make a huge difference to the ride on bad corrugations. Rule of thumb is drop to 80% and maximum speed drops to 80% as well (in practice this means 50mph). If I go lower then maximum speed goes lower as well. Duallies complicate things considerably, especially if I want to lower the pressures for a long distance, but I would be happy to air them down a bit as long as they weren't actually rubbing. Sand driving is different and then it is a matter of going as low as is necessary, or stay out of the sand.
On my OKA which has single tyres front and back, I have run them at 30psi instead of the normal 55psi for up to 2000km at a time without any problems with tyre wear, but of course the maximum driving speeds were way down as well.
All that said, a vehicle with dual wheels is the worst possible vehicle for off-road driving. Traction seems poor (which might be why my Ford has two massive tow rings on the front and nothing at all on the back) and there is the constant worry of getting rocks caught between the tyres. Happened once on the Airstream - which was obvious because of the hopping and clunking, but the once on the F350 was only noticed at a coffee stop. Getting the rocks out was a major job too.
