I have to admit that I am a bit lazy when it comes to doing laundry. I always push it until I cannot avoid it any longer. However, when I comes to new fabrics, I usually wash them right away, so they are ready just in case I think of a project to use them for.
There are a few guidelines to follow when washing new fabrics, since some fabrics tend to shrink more than others depending on the kind of fiber that is used. The general advice is to wash all fabrics to remove surplus color, dirt and chemicals and to shrink them.
Let me start out by saying that you should NEVER wash precut fabrics. Precuts like charmpacks and jelly rolls all have a consistent size. If you wash them they will shrink to various sizes, differ in shape and lose some of the fabric since the edges tend to unravel.
My second rule is to ALWAYS wash fabrics that are going to be used for garments. You would not want to sew a dress that fits perfectly and then wash it only to discover that it shrunk and you can no longer wear it. Also some garments require different types of fabric and they all shrink to different degrees.
Here is a list with different fabrics:
Broadcloth and quilting cotton: Washed and dry, I usually wash and dry on a medium setting.
Flannel: Likes to shrink a lot, which is why prewashing and drying at high temperatures is necessary to prevent future shrinkage. Use cooler settings for future washes.
Knits: Likes to shrink as well. I wash them at medium to high temperatures and dry them in dryer for pre-treatment. I do not put knits in the dryer after finishing a project.
Linen: Do pre-wash linen, since it will get softer and feel much nicer, but it won't shrink very much. Linen is usually very durable and can take relatively high temperatures.
Silk: Wash in the sink by hand with a mild shampoo and hang dry.
Wool: Only wash at very low temperatures. No dryer. Mild soap. Many people prefer handwashing or the dry cleaner.
Polyester: Fabrics like fleece, minky and faux leather do not need to be pre-washed, since they do not shrink. Only wash at medium temperatures. Dry on low to medium temperatures to avoid misshaping of the fabric.
Rayon: Likes to shrink. Wash in warm water on the handwashing setting and dry in medium temperatures. Keep out of the dryer after the project is finished.
For quilters: If you wash your quilt for the first time make sure to throw a color catcher, a special cloth that catches residual color, in the washing machine to prevent bleeding colors from ruining your lighter areas - this is advisable for quilts made with precuts.
Be sure to check which temperature your fabrics can be washed at. Most batting does not need prewashing since it either does not shrink at all or only shrinks minimally. If you are making a quilt that will never need washing, like a wall hanging, you do not need to think about prewashing and fabric type.
Protip: Sew the ends of your yardage with a zig zag stitch or serge it before throwing it in the washer to prevent it from unravelling.