Yes. The position/orientation where you find it works best!
And, whatever that is, the only way to discover it is by experimenting with antenna position/orientation.
The 2400MHz signals just do not go through any conductive material that is more than about 4" square (or in diameter for a circle), but they are often reflected off such surfaces and sometimes the results can be either surprising or annoying.
For example, trying to get wifi to work from one building to another can be a problem because walls that have fiber glass insulation with a foil backing for a vapor barrier simply won't pass a signal through. So the antennas have to be mounted at a window. And you would expect the windows would have to be able to see each other... but they might not need to!
If the windows face a building that also has metal in the walls, everything might just work with the signal being reflected instead of passing through the other building.
Typically if one antenna is oriented vertically, the other one must be also. But sometimes the polarization of the signal is changed as it is reflected off of another surface. Hence trying different orientations for the antennas is worth doing and may have surprisingly significant results (20 dB change in signal, or maybe even more).
If you are using 10 MB ethernet, go wireless. If you have
100Mbit, try to use wired connections to whatever degree you can. Is it possible to reach the 2nd floor with CAT5 cable from either the 1st or 3rd floor? If so, make those two a wired network, and the other floor a wireless connection.