According to Nintendo's Japanese website Mario and Luigi are no longer plumbers.
Mario
Mario is good at sports such as Tennis and Baseball, Soccer and Kart Racing… he’s able to handle just about anything well. It seems he used to work as a plumber a long time ago…
Luigi
Just like Mario, it seems Luigi used to work as a plumber…
Source
I guess them fixing the pipe system in Superstar Saga and that pipe in 3D World don't count then.
Mario
Mario is good at sports such as Tennis and Baseball, Soccer and Kart Racing… he’s able to handle just about anything well. It seems he used to work as a plumber a long time ago…
Luigi
Just like Mario, it seems Luigi used to work as a plumber…
Source
I guess them fixing the pipe system in Superstar Saga and that pipe in 3D World don't count then.