You can scribe any type of timber. Some timbers (like hewn) ask to be scribed more than others. It seems that it is also an advantage to scribe bents when you do not want to do the geometry needed on a scaled drawing to find placement of secondary pieces such as king post to rafter braces, etc. You can simply take the straighest and truest of all timbers and place to your suiting. Then scribe. I guess it is a question of taste, efficiency, and whichever method suits the carpenter better.