Earth, of course, is not a sphere, but for this purpose a sphere stands in adequately. We could get started on different spheroids used to provide the datum over the centuries and the continents, but it wouldn't change the numbers materially.
d.o.l.
Criminal Number 18F
the Earth is so close to perfectly spherical that it is simply not worth the effort to attempt to incorporate the mild "belling" of reality.
as you say, it would not significantly affect the data