Hanne-Grete, I usually use Hobb's Heirloom cotton (80/20), and I don't usually pre-shrink it, because I love the slightly puckery look of older cotton quilts that I get by having the batting shrink a little after the quilt is finished. This particular batting will shrink about 5%.
But, there are times when I don't want any shrinkage, so I'll pre-shrink the batting (and pre-wash the fabric) before I start. To do that, I unfold the batting and put it in a washer full of hot water and let it just soak for about 15 minutes. NO AGITATION! Then I put it in the dryer with high heat and it comes out wonderfully soft and ready to use. I do the same thing with Hobb's Washable Wool, but reduce the heat in both the washer and dryer.
Be aware that not all battings can tolerate this treatment. Read the package instructions to see if you can pre-shrink it. For instance, Mountain Mist 100% cotton batt will wind up like a washer full of wet Kleenex (tissue) if you get it wet! Ask me how I know that bit of information!!
Because most of my quilts are made to be used and loved to death, they routinely go into the washer and then into the dryer, but I am not concerned with keeping these quilts pristine and crisp and new looking. Washing and drying will make them soft and lovable. My favorite kind!