Each person needs to be researched and their last name should be the one that was used by the community that they lived in. The dit name was used to differentiate people in the community with the same first and last name. Many times the dit name only survived for one generation, but often times it carried on for several generations and frequently the dit name survives as the last name. Canadian singer Avril Lavigne has a last name that originated as the dit name Lavigne. Her family name might have been Rivard, Tessier, Poutre, Lachaise, Levasseur, Nadeau, Soucy, Bourtier, Poudret, Bernard, Brodeur and Saviot. Somewhere the family name was dropped rom useage.
I don't think it's possible to come up with steadfast rule on dit names. I think we owe it to our ancestors to refer to them with the name they would have used and responded to when they were living. The Drouin Collection a huge library of scanned images from the original parish records. From these you can see the name that was used while the person was living. Sometimes you can even find their actual signature. I don't think we should be changing someone's last name just to fit our standard's or rules.