When to use ColorMatrix

I saw someone mentioning in another thread to use this line to fix colors ColorMatrix(mode="Rec.601->Rec.709") Now my question is: when should I use this line, and when should I not? Which types of videos require it?

You use that when you are upscaling an SD video to HD and don't trust whatever players will be consuming the video to obey the colormatrix flagging from the metadata and just display as if it was 709 even if you flagged correctly.

Oh, do you mean I shouldn't rely on file metadata to determine whether or not to fix colors? I checked with MediaInfo.dll and couldn't find any information about the color matrix.