When you purchase new fabrics to have a garment made, please make sure you obtain the care instruction for that fabric.
Wash the fabric and linings and press them, BEFORE you have your garment made so that if there is any shrinkage, it happens before construction.
I always suggest you wash and iron the fabric the way you will be looking after the garment eg:
If you are going to hand wash the garment, then prep the fabric by hand washing, if you are going to machine wash then always stay with that same wash cycle. If you are always going to get it dry cleaned, then get the dry-cleaner to do the prep for you.
ALWAYS look after your fabrics the way that the shop you purchase your fabrics from suggests as they should have the instructions from the fabric manufacturer or supplier.
Clarify these when you purchase the fabrics and make a note.
That way when you care for your garment, the fit stays as nice as the day it was made
If you purchase a garment from the shops like a dress or jeans, then I do suggest you wash these before any alterations as well.
The only garments I would not wash before alterations would be the formal gowns as these have very specific care labels and delicate wash instructions.
ALWAYS look after your garments by following the care label.