I don't prewash. I *do* test suspicious fabrics for bleeding. Testing consists of dropping a small piece in a glass of water to see if any dye bleeds into the water, and also rubbing the damp fabric against some white fabric to see if any dye transfers that way. If I have a bleeder, I wash in Retayne to set the dye. If the fabric continues to bleed after two Retayne treatments, I do not use it in a quilt.
Not pre-washing saves me tons of time, plus the sizing that is left in the fabric from the manufacturer helps stabilize the fabric so I have more accurate cutting and piecing. If I do have to prewash a fabric for some reason, I will starch it when I iron so it will be stabilized like the fabric off the bolts.
The first wash of my quilts is always with Synthrapol, which suspends unset dye particles in the water so they are rinsed away instead of settling into other fabrics.
Some people are concerned about unequal shrinkage of fabrics. However, if there is sufficient quilting, shrinkage is controlled by the batting. In other words, in an adequately quilted piece, fabric will not be able to shrink more than the batting shrinks. (Fabric shrinkage is an issue with tied quilts, though, because the layers do not "become one" with the batting.)