I broke one of my own quidelines a while back -
I made "pillowcases" for my couch cushions and did not pre-shrink the fabric.
They were quite roomy before I washed them. (Maybe two to three inches of "ease.") I've only washed them in tepid water, rinsed in cold water, and barely dry them. I can barely get them back on the cushions. Great work-out wrestling them back on.
So - I know I sound like a very broken record - if you think you are going to wash the item, I think it's a very good idea to wash/shrink the fabrics before cutting and sewing them.