I do not prewash, especially fat quarters. I am in the process of making a quilt. Most of the solid pieces were fat quarters and the background was $5.00 white muslin. I piece my pattern and then quilt the block and finally put it together. While I was piecing a particular block with dark blue (my quilt is all white with various blues as the pattern) some of the blue dye stained my fingers.
I knew I had to get the blocks washed before it was put together so the blue would not fade to the white.. I zigzagged the edges and soaked all the blocks in my tub with lots of vinegar, agitated on delicates and then rewashed to get the vinegar out. I then put in dryer on delicate. I had no place to lay them out, had 72 blocks, so did dry completely.
Then I measured. The blocks were 12 1/2" when they went in and some were as small as 11 1/2" when they came out. I got out some spray starch and ironed and ironed and got most of them back to a 12" size.
Would this make be prewash from now on. No. Even with this problem, I can work around it.