What I love about gallery walls is that you can go as traditional as you want or as non traditional as you want. Usually if I am working with a traditional room, I like mixing things up and filling the wall with an assortment of colored frames and different mattes. If the room is more of an eclectic space, its nice to bring order to the wall and fill it with the same colored frames, mattes and spacing them evenly across. All in all, do what you love and have fun!!