If you're going to wash fabric first, wash everyhing first... before cutting anything. Then iron... Then fussy cut your squares, all the size you want them. Just don't cut anything until all your fabric (squares, sashing, the whole bunch) is washed.
I'm not sure if that's what you were looking for or not, or if I'm being silly and redundant with something you already knew.
The pattern's adorable, though!
(The link works if you edit out the last ")" before hitting enter.)