We receive a lot of questions if you should pre-wash fabric before cutting patterns.
The professional jeans manufacturing answer is as followed: NO.
Why?
Denim fabric has an added starch that makes the fabric stiffer and easier to sew, this gives a much cleaner result.
Second, prewashing indigo fabric can (will) cause bad wash - ripple effects in the fabric because the large piece of fabric will tangle up in the washing fabric and will cause these wash lines..
If you want to prewash to compensate shrinkage, you should do a panel wash test and measure the shrinkage; this difference you should add to the pattern.
In general, tranditional 100% cotton denimfabrics will not shrink much that it will alter the fit.. we have produced many years unwashed denim jeans, AKA dry denim, AKA raw denim jeans and they never had shrinkage issues, make sure the length is long enough so you can always make them shorter when needed. Waist shrinkage is something you can grow again when wearing.
Stretch fabrics / loose weaves and special fabrics are a different thing, you will need to do a panel test and add the shrinkage to the pattern.. there can be big differences between them.
Possibly there are hobby people that say you need to prewash before cutting, so in the end do whatever you are comfortable with,
