Today, I'm going to try to talk about profanity in fantasy writing (and cursing in general!) without using any of aforesaid naughty words. You all have permission to bap me if one slips out!
I remember very distinctly when I put the first few chapters of Flow up for critique. One reviewer pointed out that I didn't need to "sanitize" the language for young adults (I don't consider Flow a YA novel despite its main protagonist being a teenager, but that's another discussion). But Kit saying "holy schnitzel" is a verbal tic, not the writer trying to be delicate, and there are a handful of incidences of - stronger - language in the book where I felt it was necessary.
I am always conscious of the advice that dirty language looks much stronger on the page than it does when spoken. That's in part because the spoken word blips in and out of your consciousness, but on the page, it remains in your line of sight, however peripheral, until you turn to the next.
And if there's more profanity there ...
I am not a prude about language: culinary school cured me of that. I still swear infrequently enough that I can stop a room with a four letter word, and I'd like it to stay that way. Shock value is contextual. It's like using an exclamation point: every time you do, you diminish the impact of the next.
Profanity in secondary world fantasy is another beast. It takes a genuine look at the culture you're using: would these words have developed? To take a drastic example, a society without inheritance laws or that allows polygamous marriages is probably not going to place much shock-value in a certain curse that involves legitimacy. This may be personal taste, but the most popular curse word in our modern society just screams contemporary to me, even though I know the origin is pre-modern. (I learned this tidbit from my Plagues and Witches course and an instructor who was very delighted to share it.)
Other words require certain religious aspects to make sense, especially the idea that there is a place of suffering and torment in the afterlife to which one can be condemned. This is more difficult to reconcile with certain forms of polytheism. Take the Greek mythos, for instance: Tartarus and its punishments were reserved for the worst offenders, the chosen few - hi, Sisyphus! And in general, the Greek gods were less than concerned with morality. I am not as well versed in Egyptian mythology, but my understanding is that the evil were more likely to be consumed than condemned.
Inventing curse words has its own pitfalls, and I haven't really tried this yet, though I do have curses and world-specific exclamations in the more general sense. I'd rather see "he cursed" in narrative than a word out of place, whether imaginary profanity or something borrowed from the modern lexicon.
As always, it's all about using the right words.