When I was doing my Slipknot t-shirt quilt I washed all the shirts I bought. Didn't matter if they were brand new or previously used. If new they needed to be washed for possible shrinkage and if old they needed to be washed for lack of knowing where they have been before reaching my mailbox. I had a thought of a used t-shirt being worn by a really sweaty guy in a mosh pit at a Slipknot concert and that was enough to make me wash the shirt "just to be sure".
So I would wash them. You never know where its been, who touched it, or if the thift store washed them first before putting them on the shelf (which I would hope they would!)