Originally Posted by dddd

If you examine the manner that the chuck grips a Shraeder valve, it's apparent that twisting causes almost no torque on the valve stem, versus pulling it straight off which requires a lot of force if the pump head grips like some of them do (even after the lever lock is released). So for me and my particular pump, I twist every time.

I don't twist when it's a Presta valve, simply because it seems that with the much smaller gripping diameter that twisting seems to offer less of a beneficial effect on the release force. The Presta valve stems are the only ones that ever fail where they are bonded to the tube, so lubricating the threads with spit if it fits very tightly and most importantly not pressing the pump head onto the larger-diameter threads helps them not to fail.
I should mention in this regard that all Presta pump heads are designed to grip only on the smaller diameter cap threads, not on the larger nut threads. It's only when a pump head is worn out (usually from being forced onto the larger diameter threads) that it necessary to engage the larger-diameter threads of a Presta valve.