If you aren't going to be seeing any loose sand in the dunes, just the semi-packed beachfront, you probably don't even have to air down.

Here at pismo beach, we all drive in fully aired up (unless we have a trailer) all the way to our campsites. Only when we leave the beach front and hit the soft dunes to we air down.

Of course if you do air down to 15 PSI, all the small irregularities in the sand will be soaked up by the sidewall of the tires, and not your rear end.