I really think people do kids a disservice by using words in front of them when they are too young to understand the ramifications of their repeating them. If you only hang out with a very homogeneous group of like minded people, you can get away with it. But if your child goes to preschool, you are setting them up to get punished. And I think that's a bit mean to put young kids in that situation.
I've had kids be shocked and call it cursing when we say darn. I explain that darn means to mend a hole so that's what we say when there is a problem that needs mending;-) I don't mind substitute curse words in the least and think it is quite good enough that we don't actually curse.
Ds is a tween and has a full vocabulary of curse words. I don't care if he uses the occasional one if there aren't other people around (especially younger cousins who look up to him.) But I remind him he has other, more accurate, word choices. I don't want him being one of those people whose vocabulary has the f word interjected every other word. Nothing sounds stupider to me.