In the Sprint, I do not plug the vent during the flying season. However it does get plugged over the winter to keep moisture from creeping into the fuel system.
On the Flash when it had a wing tank, it was plugged because the wing goes into the wing bag and the vent is in the wing. The outlet to the tank was also plugged. never noticed air pressure to be a problem.
Now that the flash has saddle tanks, they are plugged for transport cause I do not want the fuel smell in the RV or plane trailer. Also, during the rainy season and winter both vents are/will be plugged. Again that is to keep out moisture.
It seems to me that subjecting the plane to so much heat variation that it backs off the UAT cap is unlikely. I know that we see some extreme temp changes here and that has never happened to me nor have I ever notice air pressure gushing out of a vent when I took a plug off ??? I have noticed that the UAT can contract when the fuel is sucked out of it by the fuel pump when using undersized fuel system components. Undersized fuel system components may have possibly led to a loose UAT cap for me. However, once I went to 3/16" fuel system components throughout, including the two vents, all clunks, all fittings and all lines, etc. the UAT does not contract and the UAT's cap has not loosened. Of course, my UAT's cap could have been loose from the get go.