Yes if you pay attention you can learn what works and what the numbers are for a particular combination of truck and trailer, but like Wheatking said some wonderful dispatcher can screw that up in a heartbeat!
I was lead hand at Raydan Transport out of Nisku for seven years. Yes, this is the company that invented the Raydan Air Link suspension. The original prototype was an 1988 Mack winch tractor, unit 214. My foreman and I installed this suspension into almost all 12 of our older Mack 8 ton picker units and all 6 new 2000 Western Stars.
Once we had everything set and ready, with the help of portable platform scales we tried in vain to determine pressure numbers for each unit for a given maximum allowable weight. The biggest problem was constantly changing truck and trailer combinations, but even if we removed the load and replaced it while still sitting on the scales we got different readings. we gave up on coming up with accurate numbers for each truck and would simply post a sticker on the dash with an approximate reading for the driver to use to help get it close. I remember some being several pounds different.
We were running tandem and tri axle highboys and we ran into the same trouble again. No two trailers rode at exactly the same height. We had trailers of different makes. And very few of our trucks had a consistent fifth wheel height which really seemed to make a lot of difference on the trailer end. Anything that played with the geometery of the suspension made for different readings.
So the moral of the story is, yes, if you know your unit, you can use the air pressure gauges as an approximate means of determining your weight, but like offroad said, uneven ground, brakes applied and a multitude of other factors can render the readings on those gauges useless and aint no scale master I know who will buy that as an excuse for an overweight.