Well, I had always understood that these rules were for Nobles only and have never applied them to non-nobles. On the contrary, as I believe using them for people who are not verified members of noble houses could be misleading, as it gives the impression that they were noble when they were not.
There is a noble family called Le Febvre (or Lefèvre, etc), it is the Lefèvre d'Ormesson family (the recently deceased author Jean d'Ormesson was a member of this family).. but I can assure you not all Lefèvre/Lefebvre/Lefébure are not of noble origin... Same for Lesueur/Lesieur, not to mention Dupont, Delarue, Delevoye, etc.
Also Leroux / Le Roux, Leblanc, Lebrun, Leriche, etc... none of these should be assumed noble until their noble origin can be traced.
And another point - the "de" particle is by no means a sign of nobility. Some noble families like Chabot and Talon do not have it (note that not all Chabots and Talons are related to nobility). On the other hand, non-noble families have the "de": de Gaulle for instance, or many families of Flemish origin.