I'm one of the people that strongly recommends washing EVERYTHING that is washable before cutting it - that way one knows ahead of time what the fabric will look like AFTER it's been washed - and if something is awful - I won't have wasted my time and effort on an item that would have gotten ruined by a piece of **** fabric.
Different fabrics - even when they are made of the same "ingredients" - sometimes behave very differently.
If you are planning to make "look at only" items - fabric behavior is not a particularly big deal.
If the items are going to be used and washed, I very strongly recommend washing the fabrics before cutting them.