I wash mine when I'm going to use it for a project. If I washed and ironed every piece of fabric I brought in the house, I wouldn't ever have time to quilt! :lol:
I'm very careful with reds or any fabric (like batiks) that I think might run. Those I wash with a Color Catcher (made by Shout, you can buy in in the grocery store.
I use fabric sizing (like starch) when I press the fabric. It is a wonderful product and helps to restore the feel of the sizing that gets washed out when you prewash it. Plus, it makes the ironing go faster and the wrinkles go away!