Although I don't have any scientific proof, I agree with Larry. There should be negligible change in tire pressure on or off the trailer. :P
Rolling dynamics and heat aside, tires deflect (or flex) rather than changing volume. At a constant temperature, the volume of the tire would have to change to affect the pressure. That is why pressure affects the contact patch so dramaticly and too high or too low a pressure (in a radial tire) will reduce the contact patch.
If you really want to scare yourself about pressures, try to find an accurate gage!! They are very hard to find and you would be surprised how far off your gage might be... ???
I do a little competitive driving on the side and 1 or 2 psi makes a big difference, so having a good gage is essential. Most beginners at the competition have a "bargain" gage that can be off as much as 6 or 8 psi.
Off course the bottom line is that hopefully we aren't pushing the ragged edge of the envelope and operate within the limits of our tires and the reasonable margins of error designed by the engineers.
In other words, keep up with the pressures, but don't drive yourself crazy over them
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