You could also argue that the volume a room is already on - the volume that was chosen the last time the speaker was used -  is perfect for that room. Why would you always have to change it from a very low volume?

Depending on the content, the volume level varies widely.

Same with time of day, or the genre of music, or what rooms are occupied, as well as the type of activity I am doing.

Maybe the volume of the grouped room should return to the same relative position when it was last grouped.

On first group, Room A is 60, and Room B is 40.

Then second grouping when Room A is 50, set Room B to 33 (same 66% diff)

I still would hate this if it would be standard. Maybe it could be made an option.

So… the volume in room B was perfectly acceptable the last time it was used, but now it will “immediately blast at a loud volume” when grouped? What happened in the meantime? 

You could of course simply set a daily alarm to play a silent track in room B at the required volume for 1 minute.

I would not want this feature either, as lowering the other grouped-room volume automatically would be more of a nuisance, as my rooms volume for music tend to be fairly static.. you can always ask a voice assistant to set the volume in the other room before grouping it. ‘Hey Sonos, set the volume in dining room to 20%’ or even do this via the App, or a silent Alarm, or you could use third party Apps like ‘Soro’ to set the volume in any room(s) by a chosen daily schedule