Let me answer the question of, "Should I always wash my fabric before sewing?" with a story.
Do you want to know the fastest way to learn how to cuss like a sailor?
Cut and sew a garment without pre-washing the fabric. Ask me how I know.
Eryn, how do you know?
One of my favorite parts of the sewing process is to see a box of fabric waiting on my porch. I rip open the box, twirl around with the fabric, and get started with the project.
I put in time and effort and have this lovely garment that I'm ready to wear. I wear it. Then, I wash it. Then, I try to wear it again.
Pre-washing your fabric is a necessary evil. So, yes, you need to pre-wash or pre-treat your fabric.
My advice to you is to pre-treat your fabric as soon as you get it. That way it's folded and ready for you when the mood strikes you to sew.
Not all fabrics can be washed. Some need to be steamed or even sent to the cleaners. If you're not sure how to pre-treat your fabrics, I highly recommend this book.
You can purchase this book through my Amazon store. (I'll receive a small commission for the referral). Any resource listed on Style Sew Me and in the Amazon Store are all products I own or have used, so I'm not just blowing smoke.
Other ways to gather pre-treating and fabric care information is to look at the fabric product page of the online retailer. I know lowpricefabric.com and fabric.com provide fabric care instructions.
When I receive my fabric in the mail, or purchase it locally, I serge the raw edges and wash it according to the instructions as soon as I get it. It gets dried, ironed, and folded. Now, when the perfect garment strikes me to go with that fabric, I'm ready to cut and sew. My mojo isn't spoiled by having to stop and prep the fabric.
Serging, or zig-zag stitching the edges of my woven fabrics, prevents them from fraying and warping in the wash. I definitely recommend you do this!
What other tips can you share for pre-treating your fabric?