I don't even ask the "to wash or not to wash" question. Always wash my fabrics.
If you have ever had a color run in one of your projects, then you know how devistating it can be.
My tip is to never take your fabric to your sewing room until it has visited your laundry room.
When I come home from a fabric shopping spree, I am tempted to run upstairs and let my new fabric meet my stash. But I don't. It has a bath first. There are some excellent insturctions on the internet as well as products that help with excess dyes.