I'm about to install Kahrs 3 strip planks in my dining room. The room is 12 feet wide. Each plank is 7 7/8 inches wide. That puts me at 141 3/4 inches. I need 144 inches (minus the expansion gap on either side). How/ where do I rip the boards so it will look the best?

Do you have cabinet toekicks on one of those walls? Is there a doorway you didn't account for and need another inch for?
You can rip both sides of the room if a 1 3/4 inch rip(expansion included) will look unpleasing. Or not. Do you have enough wood to waste for 2 rips and how will it all look once baseboards are installed?
Lots of things can influence that decision. If you leave a rip, try to leave it on the side least likely to be seen by the eye.
If I had two unencumbered walls and enough wood, I would split the difference.