This is how I figured out a rough method for data collection with a
BioRad MRC 600 for volume rendering with VoxelView.
We are using Nikon 60X NA 1.4 objective, 1 1/2 coverslips, and glycerol /
PBS/ n-propyl gallate mounting medium.
I expected the Z axis to be incorrect because of index of refraction
problems, but this is not what I found.
A user brought cell cultures to our facility for confocal.  One of the
stains was for f-actin.  There were a lot of dividing cells.  Viewing from
above, conventionally or w/ single optical section, cells in
metaphase appeared to be circular regardless of orientation and uniform in
size.  Thus, we assumed the cells were close to spherical.  We set the Z
step to a multiple of the X,Y resolution and interpolated accordingly
(e.g. X, Y 0.1 um, Z step 0.3 um, interpolate twice).  With 25 um diameter
cells, this takes a long time!
-Michael C.