You need to make sure that the valve stem is not so long that the Tire Pressure LED sticks further out than the edge of the rim of the tire.  This is to prevent damage while parking close to a curb or in some type of car-wash machines.  When getting new tires and valve stems we always recommend you choose short valve stems.  If you are planning on driving speeds exceeding 150 km/h (93 mph) for extended periods of time, we highly recommend the use of metal stems.  During internal testing in the lab, we were able to detect valve stem fatigue after 30 minutes of 300 km/h using an excessively long rubber valve stem (cap sticking out beyond the tire).  With short proper rubber stems, however, we did not see any fatigue, but we still would recommend metal stems if driving above 150 km/h (93 mph).