Mark Felber writes:

Reaming out the valve hole to take a Schrader valve won't hurt
anything, but don't leave any burrs or sharp edges to give you flat
tires...

Not so. For narrow rims, the stem hole is a major weak point and it
alone limits tension of spokes, tension defining the load carrying
capacity of the wheel. Spoke tension put the rim in compression and
that is limited by the smallest cross section of the rim. That occurs
at the stem hole.

Keep in mind that a Presta valve will hold air pressure better than
a Schrader valve. As for borrowing tubes when you get a flat, carry
a patch kit and a tube or two, and ytou can be the one helping
people out.

Both valve types are leak proof as far as bicycling goes. Presta
valves seal as well as Schrader valves. If there is any evidence that
there is a difference I would like to see it. That light weight tubes
have Presta valves and that light weight tubes hold air more poorly
than heavier ones does not mean that their valves leak.

Jobst Brandt