I have "damn" and "Hell" in my middle grade. They are as strong as I'll go. They are in situations where a weaker word won't feel real and the parent calls him out on the latter term in a very powerful scene. The word makes the scene. The family is religious, so it fits the characters. I'd fight for that one more than for the other one.
My twelve year old says, "four letter word, four letter word, four letter word" when he wants to curse. It's almost funny.
I curse very rarely, but I do do so, especially when I'm trying to get my teen daughter to understand how truly mad I am or if I burn myself or the like. She doesn't always catch subtler cues. (Raising my voice is nothing to her.) We've also talked about the when, where, why, etc.
I've also explained that a good vocabulary can be a huge benefit. You can use big words that your friends won't know so they think you're cursing them when you're actually saying something completely true, like bipedal juvenile. It would be hard for a teacher to give you detention for those words. One of my sayings is, "Smart people don't need to curse."