A room is never complete without some sort of window treatment, even in a contemporary interior. There are many ways to decorate a room, and each of them serves a useful purpose. You may want to have drapes that are the envy of the Palace of Versailles, or you may just want to cover your windows as inexpensively as possible. Each goal is legitimate in your mind, or you wouldn't be doing it. But, as Ewing said, you have to decide which direction is the best one for you.
Most clients start by just considering their life-styles.
If a person lives in their sweatsuit and their idea of a good time is having friends over for pizza, they will probably like casual, easy care window coverings. More formal treatments are likely to appeal to someone who enjoys dressing up and holding dinner parties. Ewing suggested a second way to find a direction is to look at things you already own. If you have 18th century style furniture, you will probably be more comfortable with traditional window coverings. If your living room features current furniture styles, then something more contemporary might be appropriate.