Stuart,
You'll get a lot of answers to the airing of the rears with duals. I took a different approach when I had a 3500 dually... I aired them to 80 psi each when towing so if I blew a rear, I could easily goa ways to find a good spot to change it. Got a little less wear on them AND a tougher ride without the 5er, but had BETTER safety edge, IMHO
It doesn't matter much for the purpose of this thread, but on my current 5500, I air the rears to 90psi ALL the time. I have enough load EMPTY to have an OK ride, but the duty cycle on this truck is about 97% heavy towing, so I KEEP it configured for it. Whenever I recover from this blasted ankle surgery, I'll install my Viair full duty cycle compressor and maybe air down some between trips for local driving, although neither me or my wife mind a truck riding like a truck. We're a truck family.
Anyhoo, just another perspective on airing dually tires. Like so many other topics on here, there are MANY that there isn't really a right OR wrong answer, just educated opinions.
Cheers, Ron