What do you guys think about the practice of leaving an open presta valve down there inside an extender? (I am aware that you can get a removable valve core and then thread that thing into certain valve extenders but I have not done so yet.) I have recently been using tweezers to simply fiddle the stem closed rather than leave it in the unscrewed position because I suspect that centrifugal force would eventually cause it to leak especially at high speed. If anyone can speak to this unequivocally please chime in.

I use the Topeak one that has a shaft that allows you to open the valve. Although I also routinely forget to tighten my valves even on plain tubes. They don't seem to leak any faster than normal.

my hunch is that the mass of that little thing isn't enough to succumb to centrifugal force (F=mv^2/2r; had to look that one up), no matter how fast you're going. but I'd be more worried about going over bumpy road.

why not just get a tube with longer stem?

Air doesn't leak out leaving the valve open. I did it for several years both ways and there's no difference. But that was before long stems. Now I use 60mm valves on one bike and 80mm on the other and no need for extenders now

Thanks guys. I am using a 50MM rim with tubulars and I didn't bother to track down long-valve tires before building the wheels. Glad to hear you did not have a problem

unequivocally NO PROBLEM I have run over 14,000 miles with the valves open on my Zipp wheels. High speeds and huge potholes not a problem.