I always use a very lightweight fusible non-woven interfacing (pellon from joannes is inexpensive & works well) - it is very lightweight (like a dryer sheet) so does not add bulk to your seams- keeps everything square and neat so no stretching. I would interface any fabric that may stretch, or fray a lot- and press the interfacing to the backs of the fabrics before cutting, definitely before any construction- since it is used to keep the fabric from stretching out of shape- give stability.