It depends upon what kind of batting you are using. If you use polyester you shouldn't need to wash it but if you use a cotton batting It will shrink a little bit. I think on a cotton batting I use it says it will shrink 3%, so you should look on the package and see what it says and it also depends upon how far apart your quilting is going to be.
You can also try making a little sampler up and see what happens when you wash it.