Back in the days when seat assignments were done at the checkin desk by selecting sticky tabs from a diagram of the aircraft cabin and attaching them to your boarding pass there were rules displayed on the diagram regarding the distribution of passengers around the cabin. After 50 years or so I cannot remember the details.
Passenger weight distribution can be important, even in the cruise. I recall a student charter flight where most of us had gathered around a guy with a guitar, who happened to be sitting towards the rear of the cabin. After a while a message was relayed from the flight deck asking if some of us could move forward, as the weight at the back was affecting the trim. But that was on a Super Constellation in 1960 and similar considerations may not be so important on more recent types.