Jeff had it right - JUST introducing outside pressure will not change the inside pressure IF the volume doesn't change. But how much the volume does change is going to vary tire by tire, load by load, radial or bias, etc.,
In my experience, normal auto tires seem to have very little difference on or off the car. My GT tire's don't change on/off. Like slowrev said, I've seen trailer tires (undersized for the load) show a difference. Still waiting to get an air/water gauge for the bigger tractor, so I don't know how/if they're effected yet. Simplest test for your tractor tires - try it both ways at least once and compare. To paraphrase Yogi - "you'll know when you know, then you'll know".