I have noticed that presta tube valve stems come in various lengths. I always assumed the longer valve stems were to allow the use of deeper aero wheels. Why is it that many wheels in print ads seem to have what I would consider an excessive amount of stem exposed? Is there an advantage or disadvantage to usng a stem that is just long enough?
I think the valve should be as short as possible while still allowing comfortable inflation of the tire.
On the other hand, a longer valve usually "won't hurt", specially if you have more than one bike or your group of fellows use to lend spare tubes to one another during rides (happens a lot to me, but in shcraeder-equiped mountainbikes).
Hope this helps!
I'd say that ideal is about 1.25" sticking out of the rim. That's enough to accommodate most pump chucks without being so long that it increases the risk of damage.
But mostly an over-long stem just looks ugly.
I've seen a tendency in the last few years for the stems on the tubes at your LBS to get (much) longer (they generally only stock one or two lengths), presumably because of aero rims. Manufacturers make more lengths, of course, but you'd have to special-order.