I pre-wash using a dye catcher and a sizing. I make my own.
Never use a pencil to mark

I usually don't prewash. I like to use cotton batting and love the look of everything shrinking together. I do tend to buy the more expensive fabrics and will prewash reds if I am using them with whites (I don't use many reds) I always, always prewash flannel fabrics because even if you buy the real expensive ones, they all shrink differently. I also use the color catchers when prewashing or washing a finished quilt. I wash all quilts after finishing before I give them away. (exception would be wall hanging that usually never get washed) I prefer working with unwashed fabrics.

About the color catchers, depending on what your washing sometimes you need to use TWO. At least that's what they told me when one outfit ran all over my brand new towels.

When I prewash I always wash the new fabrics by themselves in color groups. Blues together for instance. Thousands of bolts tells you not to wash them together, but sometimes I feel a little blending of the "blues" for instance, all going into the same quilt wouldn't hurt.

And last thing, my mom swears by putting a cup of vinegar for prewashing and then later into the load with a new quilt.

I'm not a prewasher. Haven't regreted it yet, but I can see the advantages for strong colors. I like the body and feel of off-the-bolt fabric.