On Jun 13, 11:13*am, Dan O wrote:

The idea is that the already mounted bead is free to move down into
the center of the rim, offering more slack than if the valve stem was
in the way.

Then it's in the way when the tire is the tightest. True, there is a
thicker area in the tube around the valve, which might resist pinching
with tire levers a little better, but it's pretty small.

FWIW, Park Tools "help" site starts with the valve, too. So it goes in
straight, is their point. I like "straight" OK, but my main thing,
learned 30-odd years ago when I started using Presta valves on "narrow
high pressure clinchers", was the discovery ("field operations") that
the Presta valve wouldn't take much abuse, compared to the old
Schrader valves, IRT poking/pulling/moving around after they were
mounted and didn't go on quite right-- especially poking and pulling.
So, I made the valve happy by setting it in the tire with a little air
pressure (pump, not mouth even though you can with Presta!) the same
or very close to the way it would be when fully inflated-- including a
straight valve, of course. Which helps when the valve stem is just
about long enough to work with a deep(er)-section modern rim. Although
I do carry a couple of extenders along with the camp stove and three
days' rations in the seatbag. --D-y