When I added TPMS to my previous trailer I went with the flo-thru 507's and can easily see why TST recommended metal stems (which I did). Those sensors are pretty long & I could evision them whipping around in certain situations. However after I peeled one off on a curb & switched to the older "button" style (doesn't stick out past tire) I'd be less concerned. Those are so small & light it doesn't seem like rubber stems would be a problem. YMMV.
Next trailer had metal stems so a non-issue.