I haven't done Death Valley yet but did do Hungry Valley off the Grapevine. I didn't have to air down at all. I left it in Sport Mode with traction control off to slide the rear end out. Lotso pin stripes on that trip.
Just this weekend we did the Pismo Beach dunes. I was lazy and didn't air down and got stuck. Airing down I was able to recover and then helped three other vehicles after enjoying the dunes. I was at 24 psi. A local made a comment that the lack of rain in California has made the sand much less compacted. So you may have a different experience than others have in recent years. Beach sand is completely different, of course. Traction boards, a shovel, shackles and tow strap are always a good idea. Typically, it's to help others out if you're not lazy like me.