A.Myron Ferguson responds: I often see drywall hung vertically in garages and workshops, which typically have high ceilings and walls longer than 16 feet (the longest available length of drywall). For your job, you could also buy 9-foot-long drywall, and then you wouldn't have any waster.
Another option is to use 54-inch-wide drywall, which is made for horizontal attachment on 9-foot-high walls. It's available only in 12-foot lengths, but this may work out fine if you locate the butt seams above doors or above and below windows. The 54-inch drywall is available in both standard 1/2-inch thickness and 5/8-inch fire-code thickness, in case that is an issue.
My first choice would be to use 54-inch board because I prefer taping horizontal seams. Plus, even when you include any butted seams, you may actually end up with fewer lineal feet of seams to tape. My second choice would be vertically oriented 4-by-9-foot drywall. If you install it this way, be careful not to place a seam on an outward crowned stud, and double-check your stud spacing to be sure that you won't have to cut off a beveled edge to hit a stud.
My last choice would be to hang 48-inch-high drywall horizontally. Not only does this approach generate the most lineal feet of seams to tape, but locating the seams is a problem: Seams up at the top or low along the floor are inconvenient to tape, while a double seam in the middle is difficult to hide.
For really tall rooms — like the 20-foot living room I'm doing right now — I hang the board horizontally, because otherwise it's nearly impossible to tape. It's standard to put on the stilts or work off scaffolding to tape the joints horizontally around the room, whereas trying to tape a joint that starts 20 feet high at the ceiling and runs down to the floor is nearly impossible.
Another advantage to running the board horizontally in tall spaces is that it allows me to bridge the junction between floors. I'll start with a 2-foot rip at the bottom if necessary so that a full-width board covers the joist area at the floor junction. I also avoid screwing into the band joist, because that's the most likely place for shrinkage and pops.
Myron Ferguson is a drywall contractor in Galway, N.Y., and the author of Drywall: Professional Techniques for Great Results.