Depending on how you plan to clean a garment after construction, you may need to wash the fabric before cutting out the pieces.  Some fabrics will shrink, often quite a lot, when washed.  When you purchase a fabric, take the time to look at the care instruction on the end of the bolt and make a note of how or whether to wash or dry clean.  If you do not pre-wash, it may be necessary to always hand wash the item in cold water and air dry it to prevent shrinkage.