I think I'm getting a little better understanding of using the planer. I talk to someone I work with who is a wood worker who told me that his planer has a fence to keep his planer square to the adjacent face. That's what I wasn't aware of. I assume the 12 inch planer has a fence too? I also asked, What if the beam is tapered over it's length. Can you set up some straight edges to guide a planer so that you plane the taper off of the timber? For example if the timber is wedge shaped, can you set up straight edges on the sides of the beam or at least one side so that you are taking off more material on the thick end of the timber? I'm going to borrow someone's regular power planer and use it a bit to get a feel for it. Would using a smaller planer like a 6inch be easier to handle and still allow me to do the job on a 12 inch wide timber?
Jim, you asked "Why spend the time to plane it if it's not going to show....? " I would think that you'd want a good fit between SIPS and the frame (your outside reference face). I would think this would help your was to be straight if you install drywall and paint. I would agree if roughsawn is pretty straight, then perhaps you wouldn't need to plane. But if you habe a few timbers like I do, where the original checking cause some points along the length to stick out 1/4 inch, then perhaps it would be good to plane that back down to get a good flat face to mate with your SIPS and drywall(or T&G).