The answer to this question should be based upon the specific room.  For example, in a small room adding more surround speakers can complicate seating layout.  This is because speakers that are too close to any listener can disrupt and even collapse the surround field.  We call this the “exit door effect” since localizing a nearby speaker makes you aware of the speaker and you lose the illusion of the movie created by the sound track… just like someone interrupting the movie by flooding the room with light by opening the exit door.  In such rooms, you might be better minimizing the surround channels so you can place them farther away from your listeners.  As you say a 5.1 system can be very good, however adding more channels can create a better sense of envelopment and immersion.  In most cases, I would add ceiling speakers to fully enjoy Dolby Atmos or DTS:X.  In rooms will tall ceilings (3 meters or higher), you could easily add ceiling speakers, but in lesser height rooms Dolby Atmos Enabled speakers which reflect sound off of the ceiling might be a better choice.  Larger rooms give you more options but you still must avoid having speakers too close to listeners.