They can be in either orientation, it doesn’t matter. 

Ok. Thank you. 
how about when I want to play music from the Fives. My understanding is that they have to be vertical to play in stereo. If horizontally mounted will I lose the “left” and “right” stereo sound?

No. Orientation makes a difference when you’ve got them set up as single devices in a Sonos  ‘room’. When set up as a stereo pair, it shouldn’t make any difference, to my knowledge. One will be right signal, the other will be left, there is no ‘mixing’ of the signals. 

If you have one Five, setting it up vertically will give you mono sound, but two Fives paired as a stereo pair and set up vertically will still give you stereo sound.

A single FIVE will be stereo in the horizontal orientation, however, unless you are very close to the speaker, you will not  perceive a definitive ‘Left’ and ‘Right’ stereo image. In the vertical orientation the FIVE will act similar to a point source of sound. In the horizontal orientation the sound will be slightly diffuse. The ‘best’ orientation is the one that sounds best to the listener.

In a stereo pair of horizontal FIVE’s the inner tweeters will operate at reduced level. This tends to avoid a ‘hole in the middle’ effect. In the vertical orientation the pair will behave more similar to traditional speakers. Again, ‘best’ is in the ear of the listener. If one tends to sit in a central spot while listening, vertical is probably the preferred orientation. In my opinion, if the listener is constantly moving, horizontal is ‘best’ (for me).

One can make a similar argument for surrounds. While some movies provide pinpoint audio in the rear, most are offering diffuse ambiance in the rears.

Finally, for many listeners none of the above matters and they should use the orientation that looks best. Note that this is not a negative comment about the skill of the listener -- we are all different. There are silly audio listening events that are similar to wine tasting events where it’s more about the ‘experts’ proving their prowess to other ‘experts’ than finding the best wine for you -- and the box wine in a fancy glass can win.