I'm saying virtually never, because I cannot think of a single fight that I thought a guy won by 1 round that every single round was clear cut over 12 rounds.
Robert Helenius Vs. Derek Chisora was another that Chisora clearly won 7 rounds and there was huge issues with the fight being given to Helenius because there's no way possible that he won 7 rounds.
I'm sure there's plenty more. When you judge a fight you have to treat each round like an individual fight and score accordingly and if there's no argument that can be made for the guy who "won" winning 7 rounds then it's a robbery.