In certain trying circumstances, urgent circumstances, desperate circumstances, profanity furnishes a relief denied even to prayer. -Mark Twain
I want a child who understands language, not one who is fearful of it. I am much more impressed with a (n older) kid who can swear appropriately than one who avoids certain words completely. Granted, I'm not talking about five-year olds. But there does come a point when lessons about context and discourse communities become productive. For instance, dropping a glass alone in the kitchen with a $%&#! is certainly not the same as &$^% you, perhaps not even in the same genre of communication. A twelve-year old initiating a friendly peer conversation outside of school with a "Hey, @$$#*%^!" and a high-five is in a completely different galaxy than coming home after school to a parent with the same greeting. On the parenting end, there's also the matter of context; wanting your child to know how to swear is not synonymous with wanting your child to swear.
I wish you all a damn fine weekend.