Depending on the load, 25psi+-. Much less and the chance of bending a rim increases to much.
Yup, about 10psi under factory is about where the tire gives enough to have a bigger footprint and feel a little more planted on gravel. Any more pressure than that? Doesn't seem to affect much on most tires I've run, and any less and at speed you're gonna risk a bent rim on a rock or obstacle.
In the mud/loose slop at low speeds, I'll go down to 15 if I really need to soldier through some nasty stuff...but I do so carefully.