I had not been prewashing my fabric until the past year when I started using the internet to teach myself how the tips and tricks. My conscience says I should because the teachers suggest it. I have now done both and found no difference in the finished product.
When the last stitch goes in the binding I put it in the washer in a tub of cold water with 1/2 to 1 cup of vinegar. My sister believed this sets the colors and I do it religously. I have seen some of my items years later and they may have been washed past their normal life span but the colors were close to day 1.
After a few hours soak in the vinegar water I add detergent and wash on a gentle cycle. As an earlier poster mentioned, my quilts don't have room to move around in the tub so I let the machine run through the cycles and then run it again. Sometimes the third time. Pop that quilt in the dryer on a heat setting (most people who don't sew will not know to use a low heat setting) I do turn it every 5 or so minutes so it won't get to hot in one place. If it is going to come apart I want to know it before I give it away.
I want any quilt I make and give to someone to be used.