I just filled my tires to what the Marathons said they should have. I also took the comments from the forum into consideration.
I had no problems and would suggest that if there is no dry rot present, go to what the tire suggests. Given you have a 31', I would say that you won't go wrong by following the tires suggested cold PSI of 65lbs.
BTW, I didn't mention that I saw a twin axle SOB with a blowout on my way home. It happened up the road from me realtime. Given the dual axle, it seemed very controlled and the driver of the rig had no problems getting it to the side of the road. Not sure what tires he had or pressure, but for me, 65PSI is where I was at for all 700-750 miles last week.