Effect of antenna position indoor and influence of multipath
The advice you've heard is not correct for WiFi signals for this purpose.  Multipath effects occur when the path of one portion of the signal is slightly delayed due to reflection and especially for indoor localization, the effect is pronounced.  Orientation of the receiving antenna absolutely affects RSSI for WiFi.  You can easily demonstrate this for yourself with a laptop computer.  Note the RSSI for a local access point signal, and then rotate your computer in quarter turn increments in the horizontal plane.  You will likely find that the RSSI will vary greatly.  For that reason, the typical approach is to attempt to map location based on the relative strengths of multiple WiFi Access Point (AP) signals instead.  See this paper for relevant details.