Indeed...

And--how is the resonance affected by using a tubular conductor that's
open on the ends? What if the bottom end of a monopole fed against a
ground plane (or the meeting ends of a doublet) is conical with perhaps
a 30 degree included angle, out to the uniform diameter of the tube?
Does it matter whether the upper end (outer ends) of the tube is open
or has a disk shorting across it? (A wire-frame simulation suggests
that a disk shorting the top has a small effect, but less than half its
radius.)

But certainly as Roy says, the effect on resonance is much greater than
considering the length to be one diameter longer than the end-to-end
length of the conductor.

These aren't details that are likely to matter in a ham antenna
installation, but they are interesting to me from a theoretical point
of view.

Cheers,
Tom