Interior design is subjective, but when it comes to displaying good taste, there are a few principles involved. One of those is that a couch alone can’t serve as the last word on your personal style. It needs some punctuation in the form of an extra pillow or two, preferably a matched set. Picking out sofa cushions may seem like an inconsequential afterthought, but they serve as the accent mark on any well-considered space. Here are three ways to make the most of this subtle but essential interior accessory.