You will get dozens of responses on this. Quilters are divided. Many prewash everything and won't go near a piece of fabric that has not been washed. Others wash nothing. One thing they all seem to agree on is washing batiks because they nearly always bleed. Most agree that prewashing flannel is required because it typically shrinks so much. (Unless it will be a rag quilt and then most don't wash it.)
It all comes down to read, research, listen and do what feels comfortable to you. If you don't wash, it might bleed. If you do wash, you might have lots of extra ironing to do.