Different beaches....different sand.............different pressures..............I reckon if you go in at 15 to 16 psi straight from the get go you are going in a little bit too much too early. Give yourself some where to drop to if you start to bog down.
This relates to the beach, not a desert situation, but most will run along the beach with ct on lower tides for obvious reason, so the only soft sand you are doing is either cuttings or getting to a site. In these situations you can prepare to have some speed to get where you are going so 15psi isn't really going to help to much.
Cheers NOmad.