You don’t have to; but you should.

The Five’s are most likely causing an echo effect which ruins the Dolby Atmos or any sound originating from the Arc Ultra or other Sonos sound bar when watching movies.  It is always recommended not to group speakers to Sonos soundbar for movies as there is a 75ms delay to the grouped speakers.

The delay does not occur (with speakers grouped to a Sonos soundbar) when playing a music source.

The only speakers (for optimal performance) that should be associated with a Sonos soundbar when watching movies are surrounds and sub(s).

In addition, the grouped Fives will be adding nothing to the surround effects, because they will be playing all channels, Left-Center-Right, surrounds and Atmos out of the front drivers as downmixed stereo.  So you get channels that are meant to be above and/or behind coming out of the front and delayed by 75 ms.  In short, it’s a mess, and not anything close to surround sound. 

When I am playing an Atmos playlist on my Arc-Sub-300s room the Now Playing screen shows Dolby Atmos.  When I group a non Atmos capable room with the Arc….. room there is a short muting of the sound and the Now Playing screen then shows Lossless.  When I ungroup the non capable Atmos room there is no muting and the Now Playing screen still shows Lossless.  When the next track in the Atmos playlist begins the Now Playing screen shows Dolby Atmos again.

From the above description above it does appear Dolby Atmos is changed to Lossless when a non Atmos capable room is grouped with a Atmos capable room.  It’s hard for me to tell for sure as my room is not configured to take advantage of the reflective nature of the side and height channels.

So is this is a question or intended as a helpful FYI?

For Atmos audio, the stream will be downgraded to the highest level of the least capable room in the group.  For TV sources, the first room plays Atmos, and grouped rooms play a stereo downmix of all the channels, regardless of the capability of the grouped rooms.  You could group in a fully Atmos capable room to an Atmos TV source; it’s still going to play a stereo downmix. 

So is this is a question or intended as a helpful FYI?
 

 It’s an FYI.