Collars should be interfaced for body, but only on the facing side.  If the collar is meant to stand up, then the interfacing should be stiff enough to allow that while still allowing the collar to curve around the neck.  A casual shirt collar needs only a light-weight interfacing.  Also, the weight of the interfacing should match the shirt weight; for example, a rayon shirt only needs a sheer-weight interfacing, while quilt-weight cotton needs light-weight.