We were selling hay in CO and weighing over a scale as you describe.  I was weighing on a legal scale here and they were cheating themselves about 4000 on a semi load.  I brought it to their attention so we went to their coop and the coop was in agreement with my local weight.  I always weighed locally and on his after that and his was consistantly light; we went with my weight on every load.  This guy's business was selling hay and I'm not surprised that he would be weighing light - he took us bad on a semiload of alfalfa he hauled after that, it was much lighter than the load with smaller irregular bales.  I think he was buying hay in WY because no one in CO would deal with him.  Time wounds all heals.

I would not raise the tag axle until after scaling empty even on a large platform scale; they should be stable but . . .

I always try to scale at the exact same position on the scale.  It is interesting to watch the readout as you travel along a platform scale.