well, most of us write sf&f where profanity is usually shunned. It quickly pulls the reader out of the story. Which is also why generally including profanity is bad. Course, in modern settings, it might be appropriate. Since I don't write in modern day settings, I don't usually come across this issue.
I actually disagree. Now, I don't generally like profanity, but I think 42's blanket statement does not represent the current market. Nor would it, I think, 'pull the reader out of the story.' I don't care what society you go to, there is going to be a word that is a slang euphemism for human feces. George Martin does not shy from swearing, and he is one of the current best sellers. I have rarely read a current SF book without it including some swearing.
I think you can write the book without the swearing, but the reason to do so is because you choose to avoid it, not because of the genre conventions. My favorite way is to simply say 'He swore.' It gets around the issue while acknowledging how people really act, and you can still convey emotion through other methods. I find this far better than creating fantasy swear words, which I am beginning to think is a silly way of dealing with the issue.
(That is, of course, different when making references to deity or other religious curses. These I think should be world-specific.)
I avoid swearing because I want my book to be accessible to a larger audience, and also because I believe that I should create something that represents my system of values. You have to make your own decision.