Re: Beware Metal Valve-Stem Caps!
I think the salt on the roads causes the dissimilar metal reaction and corrosion. I put aluminum caps from Wal-Mart on my Durango when I lived in NY. 3 or 4 months later (it was during the winter), I went to check the pressures and couldn't get any of them off. I was able to cut 2 of them off with a Dremel, but messed up the other 2 stems badly enough that I had to have them replaced.
I put metal caps on my Crossfire, but put some anti-sieze compound (get it at AutoZone) on all of the stems before putting them on. Now I turn them off and on every time I wash it and haven't had a problem yet.