If Umptanum is as difficult as you are expecting to find, 255 or 265 will be fine. Great for Expo type travels and exploring. If you want to play at funny rocks or moon rocks, or get into the Naches snow wheeling stuff, 285s would be a minimum width....
A little bit about airing down. Airing down gives you massively more traction, and far better suspension on the smaller bumps (baby heads would be the perfect example).
Using my 3rd gen 4runner as an example...
I believe stock I should be running 35 psi
Using it primarilly in Umptanum/Colockum/Manashtash (say 70-80 percent off highway), I run my 285/75/16 at 20 psi. Tire wear seems to still be ok, highway manners still seem ok, Off highway traction, and sucking up the small bumps are noticeably improved. I have been at this air pressure all summer long. If I was going to run to Seattle, I would air up to 30 or 35.
Now hitting mission ridge in spring or early summer, and encountering the 4 foot deep snow drifts, I would drop the air pressure to 10 or 12 psi to avoid getting stuck. I am cautious about not getting too aggressive on the gas in this scenario, both to avoid chewing into the soft snow, as well as a (possibly unfounded) fear of unseating the tire. Obviously, at minimum, you will be needing a valve core removal tool, a few extra valve cores, and a cheap (prefereably fast) pump.
It seems like the acceptable minimum for PSI is the width of the rim, or slightly above. 6 inch wide rims would have the lowest of 6 to 8 PSI. I work with a jeeper who claims he runs his tires at 4 or 5 with caution. The Naches snow wheeling guys are running 2 PSI, but of course they are also running 38 inch tires with beadlock rims.
One issue I have encountered which MAY be related to me running lower pressures is I keep losing tire weights. Something to consider.