hmm that was my thinking but a guy on a different forum said:
"Maybe this will help. I was checking the air in my tires today and just did a test for you. I set the front tires on my Silverado at 45psi this morning using an Accutire MS-5510B tire gauge.
I went out, jacked one tire up, and checked it again and got 45.2psi The sun is currently shining on the tires when it wasn't this morning so I think thats where the extra .2psi came from.
Either way I basically got the same reading with/without load so it shouldn't matter how its parked."