At least dirty talk is better than either party trying to do their taxes in their head or attempting to memorize a grocery list. You'll know when this is happening when somebody screams out "MORTGAGE DEDUCTION!" while in the heat of the moment. Personally, I'd rather a gal scream "YOU GOT A FUCKING REFUND!" when hitting the O. Then, I will have known that I can actually afford whatever it is I'm doing.
Bonus points if you can see their mouths moving during happy-time-fun-time while they're trying to solve 6th order derivatives or some such.
In all seriousness, it totally has to be spontaneous and in the moment. Otherwise, it comes off as rehearsed and awkward.