When I have worked with fleece, I have always machine washed it and dried it on hot first. Fleece can shrink a *lot* -- much more than regular fabric or even cotton batting -- so I think it's a good idea to do this.
I also starched my fleece heavily. It makes it much easier to piece accurately. For a backing, heavy starching ensures that it will not bunch up or stretch out of shape while you are quilting. After washing and drying the fleece, I starched it with a 50/50 solution of Sta-Flo and water and threw it back into the dryer. I ironed it with steam before cutting.
With fleece it's a good idea to make 1/2-inch seams rather than 1/4-inch seams. Especially for a backing, this will ensure that the seams don't pull apart with heavy use.
Also make sure to check and clean out the bobbin area of your machine frequently when sewing fleece. Fleece creates more lint than regular quilting cottons.