You can do it anyway you like. When I took my beginner strip piecing class we used Mary Ellen Hopkins' book, You Can Sit on My Quilt. She says just strip around the center square and square them all up when you are done - precision within the blocks be "darned." Or you can do it the Marti Michel way and carefully add the exact size rectangle and square up the block after you add each log. It is completely up to you and what level of detail and precision works for your project.