Well, my knowledge of the fine points is very basic, but I do know that just about any bottle will sound nicely and the only thing they have in common is a smooth clean edge. Various sizes, shapes and undercuts can have an effect on the timbre or voice, but if it's not making a sound, especially on the low octave, it might be a rough surface.
I've been practicing on a piece of straight pipe, trying to learn/compose tunes which use the "Harmonic Series" and I've really come to appreciate the effort it takes to reach each separate register dependably. The amount of energy in the higher registers sometimes allow you to pull a note out of a pipe that just won't do the root note.
I visualize it this way, although I may be really wrong on the science. I figure that the lower notes have a larger wavelength, and so it is easier for it to be disturbed by very small cracks and bumps, which throw in extra waves that you don't want. However the high notes, being exponentially shorter, can kinda slip in between the bumps, absorbing the extra waves into themselves.
It seems to be the reverse in the length of the pipe, where an irregularity can disrupt the high notes (by putting in a node or pressure point where it doesn't belong) while a long low note simply rides over a drop of condensation or speck of dust without noticing it.
Hope that helps...