This is my first attempt to model a human.  I am using Carrara 8 pro.  I don't have ZBrush or anything like that.  I've used symmetry and the extrude tool with some success.  Now that I have the basic form, I am adding details. I want to keep everything symmetric, but I also want to add spheres for the eyes.  The eyes in the pic are primitives just to see what it would look like (and the hair is Amy hair from DAZ).

If I already have a symmetric model in the modeling room, is there a way to use the  Construct, add Sphere tabs and still maintain symmetry for the eyes?

I am doing the exact same thing.  Modeling a character entirely in Carrara.  For the eyes I ended up creating a separate vertex object, aligning it carefully and then duplicte the eye with symetry.

Yes, dont use mesh for the eyes inside the base mesh. If you do that you cant animate it without using ONLY morphs. When you nested a eye ball to the mesh in the Assembler Room, its more easy to control the eye position and move it.

Thanks. I won't nest the eyes within the base mesh. Eye morphs are vital.  Are there other similar best practices.?  Teeth?  What else?  

In animation we use targets for the Eyes. So you add a Target Helper to point the eyes to the right position, thats why the eye are parented to the base mesh. Morphs in eyes are used to adjust pupil morphs and other things.

If you use to model the eye inside d emesh, you can create one and simple duplicate the mesh, in this case you will need to use bones to animate them.

Teeth can be a mesh inside the base mesh, since exopressions are based in morphs too, so you can adjust the teeths according to the morphs.

Nice job on the human female model.