i have never run a 16" wheels so i have no experiance with them.
that being said i have run plenty of 15s and 17s, in widths from 6"-10" and aired down plenty in all different types of terrain and driving conditions.
i have run as low as 4psi and never lost a bead. this topic has just as much if not more to do with driving style and conditions than it does tire size and wheel size. if you tend to be heavier footed then going below 15psi you can be asking for trouble. i have always been very cautious and thought full when lower than 12-14 psi. and i will only drop to the single digits in snow and when i do i keep my speeds under 20mph even no the flat and strait.
the lower you go the more side wall flex you get -obviously, and hence the advantages of lowering tire pressure. but with that comes greater risk of the vehicles latteral motion in snow mud or rocks to force the tire off the wheel. also the lower the pressure the easier it is for foreign debris to get into the bead of the tire -i dont like to go much lower than 10psi or so unless im in snow -mud and rocks can be a PITA to clean out of a wheel.
another thing that will greatly affect this is the weight of your vehicle. -in my 85 pick up i run 5 psi in the front and 4 psi in the rear. there is no weight in that truck and it works great(C rated tires). but when i would take my 97 4runner out i would only drop it down to about 12 psi(i got more flex at 12 psi in it than i did with 5 psi in the pickup), but it weights about 5700lbs on a wheeling trip, and i was a little more aggressive wheeling it.
so knowing how you are going to use your truck, ie what kind of conditions you are talking about sand snow mud rocks etc will help, and also are you planning on towing a trailer, or being fully loaded -5 people and gear or empty and are weighed down by armor -skids, sliders etc. it all makes a difference. a heavier truck wont need as much air removed and having more air can actually sometimes be better for things all around -wheels and tires.
but in the end its gonna come down to how comfortable you feel. i will say this though. i have driven home probably ~50mi on 12psi before and while yes it was not a fun drive and it was very sloppy i did it just fine as long as i stayed under 60mph(yes yes i know i shouldnt have gone that fast -shut up). just saying its better than going too low and losing a bead with no means to reseat it. if you dont feel going lower than 18psi is safe, then even at 18 you will have much much noticeable gains over your 35+psi hwy DD pressures.
