Never worry about valve stems again
http://www.summitracing.com/parts/BSP-999900PS, Tires leak air by nature due to molecule sizes of rubber, oxygen, nitrogen, etc. If you just use gas station/compressor air... expect 1-2 psi a month on average until all the oxygen has leaked out. If you use nitrogen, 1-2psi every 6 months. The claims about how nitrogen makes you get better gas mileage is based on the fact that people never check their tire pressure. Really, just the stuff doesn't leak out rubber as fast as the 20% oxygen in our air.
If I trailered a lot and had a spare, i'd have just the spare filled with nitrogen. Filled to 35psi in the spring, it should be 30 or so by winter.