I don't think there is one "right way" to do this.
I typically leave the trailer (7000lbs) at full pressure. My logic is there is less resistance to pull. My truck tires are however aired down. I haven't had an issue with the trailer trenching in the sand, should it start trenching I would air down.
Friends of mine routinely air down both truck and trailer. I think it's preference, the sand condition, and your tow vehicle that affect this decision.
I have learned that I prefer to roll to a stop. I find breaking causes my tires to dig in a bit.
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)