I definitely use one at night, sometimes not in the morning because it affects how things taste if I want a morning snack. Mouthwash is useful for killing bacteria and removing any debris you missed with brushing and flossing. Using it at night also helps fight that morning breath. 🙂
I use mouthwash after brushing, flossing, and using my tongue scrapper. My mouth feels a lot cleaner and I have noticed that when I keep up with this twice a day my breath doesn’t smell as bad. Gargling also helps clean the tonsil area out, which is were I build up a lot of mucus.