Coregistration using Manually selected GCP?

If you want to use coherence as a measure for change, the interferometry subforum is suitable. What menhdahl was probably trying to say is that for interferometric approaches, such as yours, manual GCPs are not exact enough, because correct coherence relies on accuracy below the pixel resolution. This can only be achieved by automated registration, such as cross correlation or DEM aassisted registration.

Areas of low coherence as in your example are not a result from bad coregistration, but more likely caused by phase decorrelation over vegetation, for example.
So actually low coherence can be an indicator for change of scattering mechanisms, or even damages.