I had quite a few comments on my last post about prewashing fabric, so I thought I talk a little about why I do this. This is what works for me, but I'd never presume to tell anyone else that they should do likewise. There are no quilt police - do whatever works for you. If you do fine with fabric straight off the bolt then there is no reason you should prewash.
Fabrics used to be processed with a lot of formaldehyde, which gave off unpleasant fumes before washing. I washed all fabric before I brought it into the house, as I got bad headaches from the fumes. The couple times I worked in a fabric store it took at least a couple months before I learned to ignore the headaches I got while working. I don't know whether they still use formaldehyde in fabric manufacture or not; I know they don't use nearly as much as they used to if it is still used, because the fumes aren't nearly as bad. I do know, however, that much of our quilting fabric is printed overseas because chemicals used are not allowed in this country by OSHA due to their toxicity. Fabrics used in antique quilts are often not available today - sometimes because the process to make them has been lost but sometimes because the chemicals needed to made them are too toxic. I prefer to wash as many chemicals out of my fabrics as possible.
I've had some bad luck with bleeding of fabrics that haven't been washed. I've had to touch up a quilt with a Q-tip dipped in bleach to take pink out of light fabrics that's bled from the reds and purples next to them. I don't want any more bleeding surprises when my quilts are washed. This is also why I sort by color before washing - I've had a couple surprises where one fabric has bled onto another during prewashing.
I've had fabrics shrink at different rates - some not at all and some as much as 4-5%. Since I use dozens of fabrics in a single quilt I don't want them shrinking at different rates because that might cause puckering in parts of the quilt.
I really do enjoy ironing my fabrics after prewashing. I take them from the dryer when they are still damp so the wrinkles come out easily. As I smooth each piece I drink in the colors, the design and the feel. I imprint each piece into my memory and dream of how I might use it. I become acquainted with each piece. The last 2-3 years I've done a lot of buying of "collections", so I don't really know what each piece looks like until I do this. Somehow my mind remembers each of them, so I've very rarely bought the same fabric twice. Books - yes - one even three times - but not fabric. I almost always sort fabrics by color for storage. When I'm doing a new quilt I decide on the colors, and then I pull 20-40 pieces of each color from my stash before beginning the quilt.
Since I bought my front loading washer I have much less trouble with strings on the prewashed pieces. I run them on "handwash" at the "quick" setting, using warm water and Orvus paste. I buy the Orvus at the feed store in a big jar that lasts many years. I think I'm on my third jar in 20 years. It's very economical that way. I trim the worst strings as I press, and I don't worry about the others.
If I have little pieces to wash - like what I get in a kit - I put them in a mesh hosery bag before washing them to protect them and prevent them from getting lost.
Washing does take away that new crispness, but it's easily replaced by using spray starch or fabric sizing as needed. Most of the time I don't need to do that. I like to use quilt shop quality fabrics, and they usually don't have the limpness after washing that cheaper fabrics do. I keep a can of spray starch ready for those that do.
I don't like to wait to wash fabrics until I'm going to use them because (1) I often use only a little bit of each fabric in a quilt, (2) I want to be able to cut all fabrics immediately when I'm creating - I don't want to wait to wash them, and (3) it would be impossible for me to remember which is washed and which isn't.
I hope this answers everyone's questions about why I prewash.