I allow my son to say dammit! I don't know if that's as unusual as I imagine, but I don't know any other kids who are allowed to get away with it, except for maybe Max's cousins. He always says it appropriately. For example...
Max: "Where's daddy?"
Me: "At work."
Me: "What's wrong?"
Max: "I got pee on my underwear!"
I've explained to him that he should not say this at school and that not all grown ups will think it's okay that he says this. I've told him that his dad and I don't think it's a big deal, but to be respectful if anyone asks him not to say it when we're not around. My husband always says that profanity is his second language. I don't swear very much, but I'm sure that Max hears a very frustrated "shit!" or "dammit!" from me from time to time. I feel like a hypocrite by using words that he is prohibited from. After all, if he shouldn't use them, then neither should we. And in the long run, they're just words. I think it's sort of ridiculous when people substitute their cursing words. I mean who cares if you say "damn" or "darn?" The sentiment is the same.