From my understanding of wireless APs, you want to equally space them out across your floor plan, to get as much coverage as you can, with as few 'deadzones', but not too much overlap. So if you had one AP only, then it would be more or less in the middle of your area. But with two they would be spread towards either end and so on.
However, what's the strategy when you are arranging WAPs over 2 or more floors in a building? Say you had just one in the middle of each floor, then that could well place them directly on top of each other. Given that they have 3D coverage, would that then give too much overlap in the vertical dimension? Or are their coverage areas more 'squashed', rather than truly spherical?
I guess all APs are different and ones with movable antennae could be fine tuned for this situation, but are there any rules of thumb, conventions etc?