I prewash and rip anything that needs to be straight. Even with my longest ruler I can't cut say border pieces straight. If I tear them I know they are on grain. Years ago I worked for a Bernina dealer who made her bread and butter doing custom dressmaking and alterations. I learned a lot from her about fabric. Straight of grain is very important in garment construction. According to her fabric is often loaded on the bolt damp which distorts it even more. In her shop we made everything from wedding dresses to shower curtains. If it had to hang straight we either tore it or pulled a thread to get the exact straight of grain. It really does make a difference in the finished product.