I like to stay around 15-18 for the Mexico beaches but like the others said there are a lot of considerations. One thing I haven't seen mentioned is speed. There are stretches of hard pack where you can easily get up to 40-50 on the beach and I don't feel comfortable being aired down too much at those speeds due to sidewall flex generating excess heat which is not good. Last year I stayed at 18 for that reason. If I ever have a situation where I need to go really low at least with the on board air I can air back up for speed.
Your weight has a lot to do with it also, With me and a passenger I'm only around 4,000 lbs. The first time I took the TJ on the beach I would get "stuck" sometimes just stopping in the powdery stuff but all I had to do to get rolling again was scrape the mounds from in front of the tires, took all of 30 seconds and I was rolling again.
In contrast, we managed to bury my 8800 lb Superduty in the powdery dunes once and it took a good 45 minutes to get it out with shoveling and airing the 275's down to 20. We were probably loaded close to 10,000 so I had to keep the speed way down when I reached the hard pack. The TJ is way more capable in the sand even aired up all the way.
How do you guys determine how much to air down if the tires are hot? Say your highway pressure is 30 cold and you want to air down to 15, and your tires are hot and reading 35 so do you let out 15 or 20?