There are two reasons the wood is run in the direction it is. First, and most importantly, the direction of the wood reinforces the importance of the view, leading the eye to the outside. Second, the ceiling wood is installed perpendicular to the roof framing. If the ceiling wood is installed parallel to the roof framing, another layer of material has to be added so that the ceiling wood has something to be attached to. Hope this all helps.

Thanks, Bud, for your quick response. Our house roof structure is scissor trusses, vaulted. My builder installed spaced wood strips perpendicular to the trusses simply to hold up the insulation, and to which the finishing T&G wood can attach to. My builder is suggesting putting the wood perpendicular to the peak line which runs maybe 35 feet over a living room, kitchen and dining area..open plan, and the boards come long enough to go from center peak to lowest wall edge without seams. My concern is that all the ceilings I see have the wood going parallel to the peak line, and I'm sure it is because they are perpendicular to the roof framing as you mention, but do you think it would look ok the other way....perpendicular to the peak center line? I would love to see this pattern in some photo.

You have to start with what you want to achieve visually. Running the boards parallel to the "peak line" will be very different visually from running the boards perpendicular. My sense is that the builder wants to run the boards a certain way because the "wood strips" make it easy to do so. To run the boards horizontally would require some new material to attach the boards to. But you have to ask yourself if that's how you want the finished room to look like. What does your architect think? He / she should be able to help you visualize the room and guide you into making the best decision for you.