I always thought it was best to have my sub between my two bookshelf speakers, but now a big screen tv has been added to the mix and I don't know what to do. Should I place the speakers in close to the TV and place the sub on one side or the other of the speakers, or should I try to keep the sub in the middle, but not too close to the tv?
Putting a sub in the corner gives you about 9db gain (meaning it's a lot louder from the corner than near a wall). Now this doesn't necessarily translate as better. That's a function of the size of the room.
As to placement tv-wise, if the sub is shielded then the best place to put it is close to the TV. This is because integration between the sub and sats is easier to achieve this way typically.
If you will enter "subwoofer placement" into a search engine, you will get a dozen or more articles detailing how and where to place a subwoofer in any room to achieve the best overall sound.