I try to have Weapons and Nav both manned when the officers don't need a rest (if for no other reason than that this ain't a cruise ship!
) Eventually, if they get decorated with the German Cross in Gold, you can leave them at their posts 24-7.
IIRC, having an officer qualified on gunnery and flak will significantly improve your efficiency (the green bar above each compartment) in those areas. My watch officer is always qualified as a watchman, a gunnery officer, and a flak gunner, in that order (although the order does not matter). By the same token, having an officer qualified as a torpedoman greatly improves the efficiency in torpedo compartments when reloading.