FiftySix

I have had valve cores leak, but it's very rare. Usually you can see a valve core fail the water dunk test by the bubbles coming out.

Of course, one could buy a set of the metal valve caps with the seal inside the cap if your really concerned about cores leaking.

Permeability of tubes was mentioned as well. I can say that in my limited experience with thick thorn resistant bicycle tubes that they hold air much longer than the standard thin tubes.