Generally speaking, lower pressure means it is more difficult for rock punctures. (Tire just deforms around the obstruction) The worry with low PSI is either wheel damage if you do not have much sidewall left between the obstruction and the wheel or if your pressure is too low on the sand and you (foolishly) make a quick turn, you can roll the tire off the wheel or break the seal instantly deflating the tire. 18 psi is exactly what I run on my truck with Nitto Dune Grapplers. I think I was somewhere around 20psi with my factory 19" wheels before that.
Sand driving is all about "float". You want as much surface area to touch the sand as possible, so the lower the psi the better. For traction / float: slicks work best, followed by purpose sand tires, followed by well worn street tires, followed by regular street tires, then A/Ts, followed by M/Ts. Muds are horrible in sand because they are designed to "dig". It's basically the exact opposite of what you want in the sand.
I off-road exclusively in the sand living here in the Middle East, so hit me up with any questions you might have.