Preshrinking batting is personal preference. Some of us want the 'old quilt' look right away (actually a lot of us) and never preshrink.
However, if you are doing a lot of fine quilting (lines close together) or a wall quilt, you may want to preshrink or you'll lose the fine stitching. Make sure the batting CAN be preshrunk (look at the the manufacturer's instructions). Shrinking occurs in the dryer, not the washer. I put the batting into the washer, and run the spin cycle. No agitation occurs. It's usually plenty damp by the time it is done. Use the delicate cycle. Then, put into the dryer on low and dry. Everybody has different settings. I usually do 20 minutes on low or medium.
I most commonly preshrink Warm and Natural.