I was told that some older labs get a paralysis of some sort in their throats. Also, I am concerned about grieving her loss, when the time comes. Older big dogs do sometimes develop laryngeal paralysis. This can occur spontaneously and it also can occur as a secondary effect when hypothyroidism is present, I think. Usually, I don't think of this as a common cause of coughing but that is one sign reported to occur with laryngeal paralysis.
Some dogs even faint from difficult breathing.
Other possible problems include heart disease, chronic bronchitis, tracheal collapse and cancers of the trachea or lungs, as well as other less common problems. If you didn't mention the cough you might want to set up an appointment to discuss it with your vet, though. It is always hard to deal with grief over loss of a pet. A friend is a friend, whether they have four feet or two. Everyone handles the grief differently.