I learned the hard way...I washed (well, in the sink) some fabric for a Dear Jane swap last year -yes, red- then I proceeded to make my set of blocks...18 were due. I had 11 made...had spent hours and hours on them. I laid the pile on my ironing board (which was damp from ironing something else) and I went to bed. The next day my ironing board was red!!!! and the white on the blocks had splotchy pink spots...I rushed them to the sink...they ran..and ran...and ran...wouldn't quilt.

I threw them in a drawer..threw away the rest of the red fabric...WASHED more fabric in the machine and started over.

YES...I always wash my fabric now...and I give Color Catchers with each quilt I give away.

I always wash all my fabric as soon as I get home from my
LQS. Washing removes excess dye and chemicals on the fabric.
Fabrics shrink when washed, but they all shrink at a different rate. If fabric isn't prewashed the fabrics may shrink differently when the quilt is washed the first time. Some fabrics have a lot of chemicals on them and is necessary to prewash to prevent illnesses like chemical sensitivity and formaldehyde allergies.

The only fabric that I pre-washed was the fabric from the Gee's Bend collection. The red is a nightmare. The other colors were not to bad. Otherwise I rarely pre-wash.

i wash my reds and blacks that is about it

I wash all my fabric since I made placemats without washing the fabric and the first time thru the wash there was shrinkage that made them look awful. I serge the raw edge before washing and that stops ravelling.

I wash my fabric. I have kids that divelope rashes from the chemicals.

I run test reds, purples and blacks all the time plus dark colors as I have had them put excess dye on to many projects. Put in hot water and squeeze a couple times, if water is clear -hang them to dry and press. Most of the sizing is still in there so they are still crisp. If they bleed, I use Retayne and Synthrapol to get excess dye out.
Just had 3 reds that bled like crazy. Thank goodness I tested them before putting them into my applique.

i normally put mine on permenant press with salt and vinegar only in my water

I didn't wash when I started, but then heard so many stories that I started washing almost everything. It gives me peace of mind. If I don't wash, I worry constantly as I'm sewing that the thing will bleed the first time it's washed and all my beautiful work will be ruined. It's not worth it.

I always at least pre-rinse fabrics to get the chemicals out and put them into the dryer to pre-shrink. If it is a red or a fabric I think will run, I place it into warm water, let it soak for 24 hours and keep changing the water until it stops running. Sometimes it is up to a week before all the excess dye is gone, then I do the vinegar trick to make sure it is set and let it soak again in warm water to make sure it will not bleed. Only then do I feel safe using the fabric.