Fabric shrinkage is usually controlled by quilters prewashing their fabrics. While this appears a good idea there are a few things to think about first.
* Shrinking your fabrics removes the mildew retardant that is needed if the fabrics are going to be stored.
* Shrinking also removes the finish that was applied to control light damage.
* Shrinking the fabrics can cause them to be more elastic and thus more difficult to manage when cutting or sewing them.
* If you decide to make a more antique looking quilt the best way to do this is not to preshrink your fabrics until after the quilt has been made.
There are several types of shrinkage that occur with cotton fabrics. Some processes are done in the manufacturing of cotton to control shrinkage. Others occur with the end user washing and/or drying the fabrics. The most common cause of shrinkage is the temperature in the tumble dryer. Removing the fabric while it is still slightly damp helps reduce this. Or you could dry the fabric on the line but this can cause other problems with color migration and fading.
Shrink tests have been done with cotton fabrics and shown that good quality fabrics shrink less, around 2% both in length and width. Poor quality fabrics when tested shrank up to 6% in length and width.
Another factor to consider is that fabrics are stabilized when stitched together in a quilt, so they shrink less than when left in a whole piece.
Whether you prewash your fabric to control its shrinkage or not is a personal choice. Remember to consider all the implications of your choice. Also the manufacturing processes of 100% cotton fabric for quilting will only improve with time.