I live in Denver and almost always cross the divide to go boating, or at least Kenosha Pass. I set mine at 2psi at home and run with it, never had an issue but I have a newer hypalon boat that has no issues, might be more cautious with and older boat.
I worked for an outfitter for while and he said boats are designed for a max of 10psi, we would fill them to operating pressure in Denver and drive to the Upper C never had a problem.
I do let air out on the way home when it is hot but I keep and eye on it on the road. I have had it loose to much pressure coming down into town before and the ass end hanging off the trailer. I am usually more worried about wear from the frame or straps so I like to keep it rather full to avoid the bouncing that it does as we move down the road.
I have seen boats that are partially inflated to the point the frame is almost hitting the deck of the trailer and I would think you would end up with the frame rubbing through the outer coating of the fabric.