It is true that all corvids will mob any bird of prey that they come across in their territory.
Whether the best time to attempt this is whilst you are decoying pigeons, is debatable.
Personally, I prefer to tackle crows when I can give them my full attention, and not be distracted by the chance of a pigeon.
Hides usually have to be better when decoying crows, as they spot movement far quicker than any woodpigeon, which means chances at pigeons can be missed, which is frustrating.
I am not a believer in crow decoys acting as a confidence booster to make pigeons more confident, so all in all, when I am after crows, I shoot crows, and when I am after pigeons, I shoot pigeons.