You may get some shrinkage, but definitely not the ultimate.
So if you are trying to match your non-washed piecing fabric,
then that is probably the best route to go.
The washing may remove the stiffness. And may not.
If not, then you may want to try another wash, and dry in the dryer.
Keep in mind, some fabrics stay stiff, no matter what you do to them!
Been down that road ... and I hot wash/dry before I start to work.
Good Luck!!