Wow this can really cause a debate. I personally wash all my fabric because the chemicals on the new fabric irritates my skin. After a while my fingernails starts to split and my hands itch.
It is a personal preference - what ever your prefer. Sometimes it is a matter of how fast you want to start the project when you bring the fabric home. Or lack of water, if you need to super conserve. Some really like the look of the fabric before it is washed, it is easy to handle too.