With the tire valve at the 12 o'clock position. I would also like to point out that filled tires will sometimes spin on the rim when the tire pressure is low, so I keep mine at 30 PSI. Another thing to consider is that the rubber tire valve stems that the tractors come from the factory with will deteriorate with age, and if at any time you are going to have the "filled" tires off of the car, you should consider changing the valve stems to metal stems.
I borrowed a tire bead breaker from a neighbor and with the tire and rim flat on the ground, stem pointed up, I was able to break the bead and push the tire down enough to get the metal tire valve into the hole. I just cut the rubber valve off and didn't worry about the part that fell inside the tire. I was more concerned about saving the solution that the dealer-installed. I consider metal valve stem replacement a necessary maintenance item to do early in the life of the tractor, and not having to think about it ever again.
