It's much like federal emissions laws in that by and large, nobody is looking except perhaps in isolated pockets.
What I mean by that is that there is a lot of anecdotal evidence saying that people haven't gotten bitten for running over their gross weight, but that doesn't make it legal.
In WA, where I spent my first 30-ish years, you may declare a gross weight UP TO THE LEGAL LIMIT, which is what the manufacturer states on the door placard or wherever it's in print.
When I registered my 2500, which has a gross vehicle weight of 8800 pounds, I could register it for 6000 pounds, 9000 pounds, or I think 12000 pounds. It would be cheaper for me to register it at 6000 pounds, and I probably would never have an issue with someone checking. If someone did check, I'd be over weight by 1500 pounds with me, a full tank of fuel, and my typical cargo. So I bumped up to the next level - 9000 pounds gross weight. If someone from the DOT were so inclined, and ran me over the scales, I would legally be in the clear so long as my truck was under 8800 pounds. 8800? Yes, 8800 pounds. That's the gross weight of the vehicle. That's the legal limit. The fact that I pay more to have a 9000 pound gross weight rating means nothing. I could pay for 12000 pound declared gross weight - that doesn't mean I could legally load up that high.
Now, when you start talking about commercial vehicles and commercial enforcement, I can't really comment. Commercial enforcement guys may look at tire/axle ratings above and beyond the gross weights.