Incredibly Acceptable
Although I try to refrain from cursing, I do think that sometimes it's more than acceptable to use vulgarities.
I mean, damn, they're a part of this bloody language and, if they were so fucking awful, then they wouldn't exist.
Sometimes, we feel things that cannot be expressed with regular words. Trying to say that you've had "a really bad day" simply doesn't cut it as saying you've had a "really shitty day." The effect isn't there. The first makes the person think "aw, poor thing," while the second really throws the punch. Without these "vulgar words," the intensity just isn't there. They just get a bad rap because they're usually associated with negativity.
Same goes for writing, since these words are best to express our own and our characters' emotions, as well as fully show our bright and colorful personalities.
No shit.
Just use responsibly.