It depends on what tires, load range, and vehicle. You need to do the chalk test and figure out at what pressure there is 100% even tread placement.
Use chalk and start at 40psi (or whatever max would be if it's lower) and go down from there. Roll your truck down the driveway and look at the tread marks from the chalk. Now drop to 35, 30, 25. By then you should be at full tread width. Usually its a safe bet around 30psi.
i have bfg at's and contacted them about this very question. they were very adimit about staying at 35psi max pressure. i used to tow a travel trailer and was thinking about going up to 40psi but they strongly recommended not doing that.
my wildpeaks are rated for 35 max. I'm running them at 32psi and they're 35/12.50/15s
As suggested earlier chalk is always a good way to test it, just make sure your alignment is somewhat decent at the time