It depends.
If the block is an involved patter, has a combination where I can't easily add the math, or has inset seams, then yes.
If taking direction from a magazine pattern that has strip piecing instructions, I want to know that I won't miscut my stash in case I don't test their instructions on a test piece.
If I am not sure about my color choices, it is a good idea to see the block made up.