Ideally place in centre of area to be covered horizontal upside down attached to ceiling.
Obviously varies with specific needs and especially building construction materials, line of sight opportunities, interfering radio/electrical sources, accessibiltiy for installation and areas of higher client density.
This orientation works well for my 25 APs covering large school over two sites: common scenario 2 APs support 6 classes arranged diagonally opposite ends of classrooms lying 3 on top of 3, in extreme case it is block of 4 on top of 4. Low client density, poss 60 clients over the 2 APs at max. Obviously they are not in centre ceiling of each class but against near walls to improve coverage into next room.
Keep in mind this is not my design, was just hired to integrate it.
Small hotel using a T5 modem as a backhaul and they want the R500 mounted to the concrete wall next to the T5. Mounting to the ceiling is not an option and the majority of the T5s are next to the bed behind the night stand. The only horizontal location is on the floor facing upward, under the night stand.
There really isn't a good solution but have been going back and forth with the engineer that created the site design on whether the wall mount (Vertical Orientation) or floor mount (Horizontal Orientation) would offer a better solution.