I wash it.
1) I want to make sure all my seams are secure, my first attempts at making quilts weren't so well constructed. I'm so much better now.
2) I want to make sure nothing bleeds.
3) I want the crinkly look you get after washing. I made a quilt for a friend. She washed it and called me crying that she had "ruined the pretty quilt" I'd given her. It was the wrinkly look you get from washing a new quilt. I reassured her that was what it was supposed to look like.
4) I have a cat and he likes quilts, especially those I'm working on. So I have to wash all the cat hair off right before I gift it. And bag it up real quick because that stupid cat can find a freshly laundered quilt so fast and make a bed in it in a flash.