Can you tell us what brand cotton batting you got? Or if it's off a bolt, did the shop include a piece of the instructions insert? If the brand can be researched, we can see if it has a scrim, which might indicate whether pre soaking and drying is a possibility.
Personally, I don't prewash my fabric, unless it is flannel. I have always thought that if I ever want to make an "art" quilt, for show more than for comfort, I would pre-soak and machine dry the Warm and Natural batting I use.
The modern quilting fabric we use today doesn't shrink much. The batting will shrink about 5% or so (Guessing). If a person doesn't have anything better to do, she could do a practice swatch, measuring a piece of batting, and hot water soak and dry it, then measure again.