Decide when the fabric will be washed
Fabric can change size when it's washed, so make your prewashing decision before planning a tight cutting layout. Follow the fabric maker's care instructions and your pattern's guidance. If the fabric is already washed and pressed, measure that piece rather than using its original purchased dimensions.
Measure what you have after washing
For fabric on hand, the washed usable width and length are the numbers that matter. Exclude frayed edges before entering the width in a calculator. If you're buying new fabric and want an allowance for shrinkage, base it on what you know about that fabric; there isn't one percentage that fits every material.
Keep extra fabric separate from seam allowance
Seam allowance is used to join pieces. It isn't a place to hide extra fabric for washing or cutting mistakes. The binding calculator has an extra-fabric percentage for a separate purchase margin. That percentage simply adds to the cutting length; it doesn't predict shrinkage or guarantee a particular size after washing.
Check the width as well as the length
Extra purchased length won't solve every shrinkage problem. If a piece becomes too narrow to fit the planned cuts, you may need more strips or a different backing arrangement. Rerun the relevant calculator with the measured usable width before relying on the earlier cut list.