I say that it is a unique case because people are different sizes and have chairs, sofas and settees of differing heights. The rule of thumb for wall mounting a LCD television is whatever is comfortable for you when you are seated. You don't want to be looking down or (in particular) looking up at the screen as this can cause strains and pains in your neck.
So sit down on your couch, not bolt upright but how you usually sit on it and look straight ahead at where you are thinking of putting the TV. Then either remember where dead centre is or rope in someone else and direct them to mark on the wall with a pencil where dead centre is located.
Then depending on the size of the telly you need to make another mark above it. This is because the initially mark will be where the absolute centre of the television should reside. This will usually be about 48" off the ground for most people.
Another thing to bear in mind is that if you use the screen mainly for gaming with motion sensor hardware, namely Nintendo Wii, Xbox Kinect or Sony Playstation Move and therefore stand up while playing, then you may require the LCD Television to be mounted higher for the reasons outlined above, to minimise pains or strains in the neck.
Finally ensure that you use an approved bracket for the make of television that you own and that it is strongly and safely secured to wall.