Hi Linda,
It's difficult to say what the lowest useful magnification is, but in
general, low magnifications are more useful then high magnifications for
conchological work.  I have a zoom stereomicroscope with a continuous range
of magnifications from 7X to 30X.  I most often work with magnifications
from 7X to 10x.  However, I do not specialize in microshells.  If I did,
then I would spend more time between 20X and 30X.  Magnifications higher
than that, in my opinion, are not very useful for shell work.  After all, a
magnification of 30X makes a 2 mm shell look like it is 60 mm, or almost two
and a half inches!  The single most important thing to keep in mind when
looking for a microscope (or any optical instrument for that matter) is that
the resolving power, or resolution, of the instrument is far more important
than the magnification.  You will see far more detail with a high resolution
20X system than you will with a low resolution 60X system.
Paul M.