Wi-Fi Myth: Point Your Antennas Perpendicular To Each Other
Source: LifeHacker and others.
Radio reception is maximized when both the Access Point and client (laptop, smartphones, tablets, etc.) have matched polarization. Some clients have vertical polarization while others have horizontal. So, pointing the antennas in both polarizations will improve performance.
With 802.11abg networks this may have been the case, but with newer n and ac Access Points (wireless routers for home) this is no longer the case. While it is true that different devices have different polarizations, the newer 802.11n/ac/ax Access Points utilize a technology known as MIMO (Multiple-Input Multiple Output) antennas.
In the past reflections of the signal off of walls, furniture, and other obstacles around the house could hurt the performance of Wi-Fi networks, with MIMO however the reflections are advantageous as the signals arrives at the receiving antennas at different times. Prior to MIMO, the difference in arrival times would cause interference for the receiving signals (talking over eachother). With MIMO, using advanced mathematics the Access Points can use multiple antennas simultaneously to improve reception.
What Does This Mean?
Point the antennas based on your manufacturers recommendation. If no recommendation is provided, have all antennas parallel to each other or at most 15-30 degree offset.