Tire pressure monitoring system dumb questions

1. Leakage is likely unless you use steel valve stems. When a rubber stem developes a leak, the tire must be dismounted in order to replace the stem.
2. My TST batteries usually last a couple of years. They are easy to replace.
3. Each sensors is "married" to a particular tire when you first set up the system. That's something you do only once.
4. Speaking for myself, I don't have a sensor on the spare.