I wash all my fabric for 3 reasons:
1. Remove the sizing. You would be amazed at all the weird shapes your fabric may end up in once the sizing is removed.
2. Wash out all the "bleeding" color. Use a color catcher in your wash or white vinegar, both will keep the color from bleeding onto other fabrics and the vinegar will actually "set" the color after the bleeding.
3. Wash out the formaldahyde. That is what fabric is treated with to keep the bugs and moths off it while it is being stored for distribution. Nothing likes fabric more than a bug.