I've smallish worldspace with no creatures or NPCs other than some game animals. It is about half open fields and half wooded areas. How closely should I navmesh? Could I mostly mesh the open fields and just go around the clumps of woods? I don't expect combat in this area just some deer hunting. It would surely save some time. What do you do with similar areas?

Couple of things.

Are you creating the mod for your own use or is it something you plan on releasing?

I believe for the Deer to travel into the wooded areas it needs to have navmesh. I completely navameshed the wooded areas in my mod to give it a more realistic feel so creatures can travel through those areas.

If you are planning on releasing your mod if users bring followers they will be able to follow the player through those areas also instead of going around to meet up with them on the other side.

Believe me I know navmeshing a wooded area is time consuming and can get quite tedious but the end result is worth it.

Have the ck generate the navmesh for you. Then take a look at the result and make corrections to your taste.

For open fields and smooth wooded areas CK does alright if you tune the generation values and try a few times. Will certainly need to be cleaned up by hand, but no need to do it all from scratch.

If it is half done can you have the Ck finish it or will what you have done be lost?

Generation is done on a cell by cell basis. It either wipes the navmesh entirely and generates from nothing or doesn't do anything.

EDIT: If you generate using a region you can specific whether you want it to regenerate hand-edited cells, or ignore them. So you can put one big region in your world and tell it to ignore hand edited cells. Finish those up yourself, then clean the ones it does generate in.