Our interest in valve stems was born early in our coach ownership when we had to repair a flat tire. I remember hearing a pop followed by whistling and decide to pull into a nearby parking lot. I had assumed we just had a flat and were going to have to buy a new tire. When the tire man showed up he reported that our valve stem had not been rated for above 65 psi. It was the rubber valve stem that had popped and whistled while the air left through its remains. We were lucky as he had a new high pressure valve stem, but not a new tire in our size.
As we are pushing the maximum rated weight in our 2008 model 241 coach, we need to run close to 75 psi in our rear tires. So we began to insist on high pressure valve stems whenever getting new tires.
Then, we purchased one of the tire pressure monitoring systems that have their sensors embedded in replacement valve stem caps. If you have such a system or may get one later, you will want metal valve stems to support these heavier caps spinning at highway speeds.
Some people use valve stem extensions either to raise the valve stem through a wheel cover or to add a bend to help fill the rear dualies. I have encountered many problems with any of these extensions working lose and causing a loss of tire pressure. I have removed all valve stem extensions and have not lost pressure since.