Hey,
the physical shape of the earth is probably best resembled as a geoid (equipotential surface approximating mean sea level)...
For Nav Systems an ellipsoid model is used (e.g. WGS84), since the assumption of earth being a sphere is much too inaccuarte to navigate precisely. The shortest distance between two points on the suface of the ellipsoid is called geodatic line. (The same as the great circle on a sphere)
Spherical geometry on an ellipsoid is really not something you want to get involved with
Trust me.... ;-)
LF and MF follow the curvature of the earth at least to some extent, due to refraction (lower air density in the higher atmosphere) and diffraction from the ground.
VLF uses the surface of the earth and the ionosphere as a wave guide, so there is some truth in your statement about great circular propagation of radio waves. Everything upwards of MF frequency range definitely propagates in the line of sight.
cheers