The upper line of fabrics are 'cured' in a hot steam rm, sealing in colors, preshrinks fabric, is what I was told at a Quilt Retreat Sharon Shamber taught the class. Moda, Hoffman, etc. The other fabrics are not heat/steam sealed.
Again, 50/50 with most quilters, most fabric swaps want pre-washed as you don't know what your getting. Lower end fabrics always prewash. But if you want that antiqued, loved look, quilt first, wash afterwards. Some will spray starch constantly pre-washed or not, keeping fabric crisp for tidy 1/4" strait seams, and lay flat finished seams.
I agree also, depends upon the project. A baby blanket getting washed a lot, I'll buy blends, they wear longer, fade less, or a toddler blanket. Cottons for 99% of my projects. Prewash high fade fabrics for color run, reds, dk blue, black, hot pink. I also use Woolite for black or color, helps seal/save from fadding, and Quilters Soap for very nice projects. When folding quilts, do not fold in halves, but in 1/3s and refold if in storage, some do every month when cleaning, or quarter, or special day of year. Some roll on big fabric tubes you can get free, put a sheet over it, store them under a bed or closet floor, no fold lines. Never store in plastic, let fabric breath. Hope it helps