Its true that aircraft haved to be trimmed during flight for weight changes
The aircraft is trimmed before flight. If you are out of trim, the aircraft will be nose or tail heavy. This could result in something as extreme as failing to take off if you got it so wrong, or lifting too early if you got it tail-heavy. Trimming before flight ensures the operating crew have the load (including fuel load) within their operating parameters for that sector.
So yes – distribution of passengers can be critical, but probably not that critical. However, the cabin crew don’t make those decisions and probably have broad ‘standards’ for seating ie nobody forward of a certain row. Safety margins are large in aviation, for very good reasons.