Syncing N1 should make the most sense because it's the minor N1 residual imbalance that creates the heterodyne or "beat" frequency in the airframe.
However if N1 imbalance is so low that there's no perceptable beat, sync whatever parameter you like!
But it seems to me that other aircraft systems (hydraulics, inverters, ???) may create more cabin noise than anything else in cruise or especially approach.