I wash EVERYTHING washable before cutting it - because I prefer as flat a look as possible.
Much as I am unenthused about starch and sizing, when cutting triangles, it is a good idea to starch/size the fabric before cutting it.
I did learn - the hard way - that it is better for the treated fabric to dry a bit before trying to iron/press it.
When soggy wet, it can stretch all over the place.