If you want a pretty complete picture of your total weights, then I would weigh once while hooked up and once with just the truck. However, when you weigh each time make sure you are loaded for travel/camping.
I don't believe in spending 45 minutes behind a truck stop getting both my weigh-ins done at once. When towing, I will run across their scales after getting fuel, go in and get my printed ticket, and pay the $10.00.
Then within the next week or so when I am camped close to another Pilot or truck stop with Cat scales, I will fuel up the truck and run across the scales solo, go inside and get my ticket, and pay another $10.00.
I may end up spending $20.00 total but I just don't believe in messing around with unhooking and hooking up in some truck stop parking lot. Seems like a waste of time to me in order to save $10.00.