I think minimizing all reflections should be your goal. Think of the room where they master the sound. I've seen one of these rooms at a company my uncle used to work for (POP Studios in Santa Monica). They pretty much deaden the entire room so any sound you get comes directly from the speakers. I remember seeing a lot of anechoic acoustic foam on the walls.
With that said, there's plenty of things you can do to your walls to deaden them. Acoustic foam is the least pleasing to the eye. A popular choice for home theaters is thick velvety "movie theater style" drapes along all the walls. Another option is thin acoustic panels, which resemble drywall wrapped in fabric.
