Profane language it's never very good, but the gravity really depends on the context.
A good rule of thumb should be: don't write anything you wouldn't want your mum, Carl Sagan, Phil Plait or your next employer to read.
It may sound like a hyperbole, but I know for certain that some notable skeptics have been browsing the site, so it's actually a real concern.
- "WTF? I clicked on the link and a virus warning came out!" this is bad form, but probably passable
- "I think these books are s**t" this is not OK at all
How to react to bad language
When anybody sees this kind of language:
- if in a post and one has edit rights, edit and remove;
- do NOT post "stop being rude" comments;
- if editing is not an option (comments or not enough rep), flag for moderation attention.