In reality, I'd guess Lois swears more freely than Clark, but she wouldn't swear a whole lot.
Oh yes she would! <g> Remember the Wedding Arc? Perry reminiscing about that ski trip where Lois broke her ankle and confiding awe-struck to Clark 'the language that came out of that woman's mouth...' as they carried her off the ski slope?
A good compromise between injecting some reality into your fanfic, Anna, and not going over pg13 is to have your character start to curse and be cut off or cut himself/herself off in mid word. That one works pretty well.
But that would apply to Lois, not Clark, I think.
I agree Clark would curse fairly mildly - he did use 'oh, god' in the show several times - mostly in the latter seasons - but that was as far as I remember him going.
LabRat