I would personally say that it strongly depends on your image. Speckle filtering can surely increase image quality and it is done best at an early stage so speckle is not falsely propagated in ongoing resampling processes (within terrain correction, for example). But if you are interested in smaller structures or image texture, speckle filtering might remove the information you are interested in.
So saying speckle filtering must be done is surely not correct, but it is advisable in many cases.
If you do it after terrain correction you have to notice that your image was already resampled to a new resolution and coordinate system. So your pixel values were already changed (and in case of bilinear resampling this involved a first smoothing). So the results might be different but it is advisable to compare.
Please also note that speckle filters perform different when applied on the db image.
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