For me it really depends on how much of the person you see in canon...does canon really say they don't smoke. There's a lot of Torchwood fic with Ianto smoking for instance. Technically he doesn't in canon. But at the same time you never see him outside of work in canon so it's possible it's just something you don't know about him. If hte show centers on someone's personal life though then it'd be harder to buy...