One does learn as one goes.
Because I have had some "surprises" in the past about shrinking, bleeding, and fiber content, I am "a bit" OCD before I get around to actually cutting fabrics.
I do burn tests on fabrics i do not recognize. Some rayons still fake me out.
Soak in hot water, then wash and dry gently.
But - some have said they have never had any problems whatsoever using unwashed fabric.
I have. I have decided it is easier to do prep work ahead of time - even though it sometimes dampens my enthusiasm a lot - than to deal with a mess later. That did not entertain me.