One thing to think about - I don't think special meals can be moved between cabins (except maybe WT/WT+?), so if anyone in your group wants a special meal, you might want to pre-book it based on who intends to sit in which cabin.

This would be an additional reason to mention a seat change to the crew because they'll sometimes come round before the meal service and check that you did in fact order a special meal, so if they're looking for Ms XXX in 42E and see Mr XXX, it might cause confusion that could be avoided.