I do use liquid soap, but only about 1/4 the amount suggested in a cold water wash, then I use a gentle cycle and cold water rinse-twice. If it had a dirty spot, I would spot clean. There are packets or sheets, they call color catchers, you toss in the washer, and although I have not used them, I am going to buy some for this quilt! I tumble my quilts in the dryer but remove slightly damp and hang or drap across 2-3 chairs and allow them to finish air drying.
During the construction of this quilt, I have sprayed water, and ironed every piece, front and back, so I don't know if that helps set the color...but I am curious to know if the heat and water does aid setting the color???
Generally, I do not prewash my fabrics, unless I am swapping. I prefer to work with new fabrics, as I like the way they feel...:)
BTW, I have a large capacity front load washer and dryer.