In ranked matches I always perform. In Koth, it depends, If I fight many times the same opponent, I stop doing the fatality after a couple of fights because, well he got the picture. Unless it's a stage fatality, I always perform them.
i never get tired of using fatalities but i like to use sub-zero's x-ray every now and then because you can hold it xD so the opponent is like oh crap now you gotta show off FINISH ME ALREADY and you're just posing and then bam ^^
on a side note. sometimes the win animation is cooler than the fatality *example : stryker* so i tend to shoot ppl or sth at the end just to see his win animation.
It really depends. If I had a close match against a good player I might as a sign of respect. If I'm playing someone whose talking trash or just generally being a jerk I just jump around them and have Kitana do a little dance by stance changing. Seems to get them more annoyed then just doing a fatality.