I usually don't prewash my background fabric, mostly because I use Muslin and whatever shrinking that happens is so negligible that I'm not worried it's going to distort the quilt.
Everything else (unless it's a jelly roll or charm pack), however, does get prewashed and I only hand wash unless it's bigger than two yards, then I machine wash.
Every quilt I've ever made, I always send out a "how to care for" sheet of instructions for the recipient to follow and whenever it comes time for them to wash it the first time, I always go over and help them. I don't want them to freak out if they wash their quilt and pull it out of the washer to find the colors have bled, if I'm there to help them, we can quickly solve the issue. I also send a small 'care package' with the quilt, small bottle of laundry soap that I know won't damage the quilt and some color catching sheets. I try to cut any bleeding off at the pass the best I can and give them as much information as I can so they can rest easy knowing they're doing things the right way. We put so much work into our quilts that I feel if we give them the right information to care for them, they'll last much, much longer and eventually become an heirloom.