Users should avoid unnecessary profanity.
There are some exceptions where it makes sense to use profanity. One example of this is when the only way to reference something that isn’t profane accurately is to use profanity. Example: What is the Greater Internet Fuckwad Theory? The guideline on profanity on Quora is mostly about tone and common sense.
If you scroll down quickly from the answer by Bodnick from 2011, to the exchange between Dan Holliday and Gigi Wolf in 2013, you can get a chuckle.
If you dig, there are some answers saying that profane language will be sanctioned (Tracey Bryan’s answer to Why aren’t rules guiding the use of profanity enforced more on Quora? from 2012), and a lot more answers recently saying “fuck no, hahaha”.
My own impression is that any rules against profanity are currently enforced either selectively or infrequently. I have not seen the 2011 guideline explicitly rescinded however. And “unnecessary profanity” does not only refer to hostile profanity:
Expecting complete strangers to understand when telling someone to ‘f-off’ is an insult or friendly banter doesn’t make sense on a site as big as this. Expecting moderators to read a long thread and try to judge the tone is not reasonable, judging sarcasm online is notoriously difficult without context.