You should use plastic or rubber caps on TMPS sensors. Using metal caps can cause corrosion.
Problem: Metal Valve Caps.
While the bright and shiny chromeplated or metal valve caps that you see everywhere are often a nice effect compared to dull black plastic caps, when it comes to TPMS stems, they're a clear and present danger. Metal caps can easily cause corrosion on the threads of TPMS stems as well as beginnning to corrode themselves. The result is all too often that the cap will rust-weld itself to the stem, leading to outcomes from the cap simply seizing when a technician tries to remove it, to crossthreading issues, or even having the stem break off entirely. When the valve stem is a nonremovable part of the TPMS monitor, this goes from being a minor issue to a critical and often expensive problem.
Achey Breakey Parts: TPMS And Corrosion