The only fabrics I prewash are flannels and very dark colored anything else. Flannels only for the shrink factor. Someone told me when I began quilting 10 or so years ago, that today's fabrics are usually stable...and to trust them.
I found this to be true. I like the crispness of unwashed cottons...they cut better, are less fray-edged or distorted, and I hate when they turn out limp from prewashing. Several pros do not advise prewashing....the battle has been going on for a long time.
I also find that washing my quilts in cold water, delicate, and damp drying them in the dryer, then gently reshaping if necessary works just fine. I do small things...lap quilts, wall hangings, and the like and have never had a problem with not prewashing.
D