I agree with your statement on the 19.5's. When I had them on my SRW truck, I took them down to 50 PSI to cross some soft sand, but did not dare go lower for fear of loosing the bead. I have no idea if I could have reseated it with my 10 gallon tank pressurized at 100 psi, but that was not the place to test it. At 50 psi, I had little squat in the sidewall but feel it did flatten out the 16PR tread a little. Laying out boards and carpet helped more than letting the air out.
Regular tires definitely benefit from airing down. I run my off road toys with .5-1.5 psi on sand verses 10 psi on hard pack and can go so much better.
