I always wash for these reasons:
I want to see what "my baby" looks like when it's done, and how it will look for years to come, before I give it away.
I love the soft, crinkly texture of a washed quilt.
Washing hides a LOT of imperfections! I recently made a wall hanging that was in a guild challenge. I finished it at 2 am the night before the competition. There were several areas in the quilting that I was unhappy with, but because it was so late I gave up and called it a night. I washed it lightly to get my quilting lines out, then dried it. It came out beautiful and I could not find those quilting imperfections.
p.s. I won the challenge!