I have always prewashed my fabric for quilting, because I learned to sew clothing first, and you always prewash the fabric for clothing to avoid any unwanted surprises the first time you wash a garment, like the sleeves shrinking up and exposing your wrist bones, or the rear becoming suddenly too tight.
Some quilting books say to prewash, some say to pretest (make sure the different fabrics shrink at the same rate and don't bleed), and some say it isn't necessary at all. So I've been sticking with what I know and prewashing all my fabric as soon as I bring it home.
Recently I was saying something about the difficulty of keeping pre-washed and non-prewashed scraps straight (when some are given by people who do not prewash) and our preacher's wife, who knows a lot about quilting, told me it really wasn't necessary. That mixing the two wouldn't matter. Even though she knows a lot about quilting, I was wary of trusting that advice to the extent of a whole quilt, so I made little miniature test quilts that mixed prewashed and non-prewashed scraps, and washed them on warm then machine dried them. The one on the left is before, the one on the right is after.
It really doesn't seem to make much difference, with quilting cottons. Mixing flannels that aren't prewashed with cottons (prewashed or not) would be a whole different story, I think, as flannel can shrink a lot the first time you wash it.
Also today I made another "Moon Over the Mountain" quilt block, although I prefer to call this one "Hippy sun". I used my little tie-dye swatch to make the sun, and then for the sky I traced the template onto tissue paper and used it as a foundation for the string piecing, then tore the paper off. It's similar to an example from Georgia Bonesteel's book, but a lot brighter colors (and wider "strings").