There are different groups of quilters. Some just use fabric straight away just iron if necessary. Group 2 don't wash but spray starch while ironing. Group 3 wash and dry and iron. Group 4 wash dry and starch before ironing.
i think these are the different groups, the starching depends some light spray while others make the fabric paper stiff. It is up to you which you prefer or have time for.

I don't prewash... ever. If I had to prewash, I'd never quilt. I hate pressing all that fabric. If there's a questionable fabric, I do a test with a small piece in some warm water and soap. If it bleeds, I get rid of it.

When I started quilting about 10 years ago, I was taught to wash and iron all my fabric. Then one day I picked up a pack of my fat quarters I had ironed and had to iron them again as the folds were so tight. That was the last time I ever washed all my fabric. If I think a fabric might bleed, I cut off a snip and water it down and put it between a white paper towel to see if it will bleed. I also use a color catcher when I wash quilts.

This is one of those questions that will be with us forever. Some of us prefer to work with the fabric with is 'stiffness' while others are aghast at the idea of not prewashing the fabric. I think it stems the fact that fabric used to shrink so much (but we also use to wash everything in very hot water) and from bleeding from the reds and indigos. If your top shrinks a bit, it just creates the slight pucker look that I like with a quilt. The bleeding you can usually resolve by washing the finished quilt with color catchers. The only time I would prewash is it I am mixing cotton with a blend. Then I don't worry about one area puckering and another not.Bottom line is do what you are most comfortable with. Over time you will develop your style. The most important thing is that you have fun and not make it seem like a chore or like work!

I wash everything. I want to get rid of chemicals, make sure the fabrics don't bleed, and to help with shrinkage.
To wash-or not- is your personal decision. :)

pre washing does not guarantee the fabric wont run....it will just run less.

I always wash with a color catcher. One of my first quilting and sewing mistakes came early when I was first starting to quilt with heavy bleeding and a dress I made that shrank so bad it was thrown away. All the chemicals they put in fabric bothers me making my hands itch and my eyes water so I have to wash first to be able to work with the fabric.

When I was just beginning to quilt I would automatically put all of my new fabric into the laundry basket. I would wash the fabric as I intended to wash the quilt with warm water and detergent and then I would machine dry on low. It seemed to take me forever to starch and iron the fabric before I could cut it. Now I don't wash the light colors but I still wash the medium to darker colors and I use color catchers now. If your fabric should bleed don't machine dry it. Put it into the washing machine with a box of baking soda and warm water. Agitate for a few minutes and then let it soak overnight. Run the washing machine through its regular cycle in the morning and your fabric should look like new. I have used this method for almost 30 years and it works every time.

I always prewash unless its a wallhanging, also!

I have not ever prewashed but having just attended a lecture with Pam Buda of Heartspun Quilts http://heartspunquilts.blogspot.com/...h-that-is.html
I just starched all of the fabric for my next project to see if it makes a huge difference.