I wash every one - well, so far the only one not washed is on my bed. I bought a new washer and dryer this spring, and looked for ones that would work well for washing large quilts. I washed a queen size yesterday, and have king to toss in this morning. I'm getting caught up on binding.
I have cats and that is the last step to remove as much cat hair/dander as possible. From that point they are kept in a closet until they find their new home.
It also gives me the opportunity to check for problems. So far I've had one that bled, even though I prewash all my fabric. I was able to clean it up and wash it again a few times. I tell the people who get them that they've already been washed, and that seems to relieve a lot of concern about how to care for them.