I used to simply spray the quilt with water until I read an article by Candy Goff on blocking. She used to have a website that was a wealth of quilt information...sadly, it's been taken down. Anyway, she washed her quilts because it helped "release" the batting so the quilting stood out better.
I can't say I've noticed much difference, except that washing gets rid of the marks. If you're machine quilting you may not have that problem.
Janet