wallboard installation

My BIL ( he is a professional rocker) says the seams are less visible when the rock is Horizontle

It also saves the mudders time as they don't spend as much time up and down a ladder or on stilts.

Running the boards horizontally is pretty much standard but not mandatory. This from the USG handbook: "Proper planning achieves the most effective use of materials, elimination of unnecessary joints, and the placement of necessary joints in the least conspicuous locations. One gypsum board should span the entire length or width of the wall or ceiling, if possible. By using the longest practical board lengths obtainable, end joints are kept to a minimum. Where they do occur, end joints should be staggered."