It depends on the batting and the look you want after it is washed. if you a using a polyester batting , nope donot prewash as it will not shrink. If you are using a cotton or cotton blend , it will depend on wether or not you want it to have the "puckered' look after washing. I have pre- shrunk for particular quilts. If you do this all of the fabrics should have been preshrunk including the backing.
If you choose to preshrink , soak the batting for at least two hours so all the fibers are completely saturated. I do spin mine ( never had any issues with washing machine problems) , then put in the dryer. Do reposition the batting a few times in the drying process. It is the heat removing the moisture that will shrink the batting.
If you are going to have it long-armed , save the package from the batting. Press the backing. ( long arm professionals hate when there are wrinkles, some will tell you if there are wrinkles , they do not gaurentee they will not still be there once quilted). Make sure your top is free from stray threads on the backside as well as the front.. They will not clip your loose threads. Its a shame to have a beautiful quilt with dark threads showing through a light fabric.
Consult your longarm person as to exactly what they require.
I typically choose to provide my own batting , as I can typically purchase cheaper, but they ( long arm person) can offer great guidance , as to the getting the right batting for the look and use you want.