I use a zip cover in winter (been using my Little Shop of Hammocks Warrior in all 4 seasons now.)

I keep the part zipped near my feet so my top quilt stays in place and open up near my head to help venting and avoid condensation.

EDIT: Just to clarify, the "zip cover" on my hammock is a mesh bug net, not a solid top cover. Temperatures here in the winter can flirt with 0 degrees Fahrenheit but are generally in the teens to 30's in the winter on average I'd say. I don't feel the need for a solid top cover, but, then again, I do have a Hammock Gear full length underquilt rated to -10 degrees...