Is controlling the centre only important in the opening?
Yes, evrey opening move is about center control, some with pawns other with bishops/knights..
Try to think about squares when you chose your opening moves, which squares do you want to control, wich do you want to awoid your opponent from controlling. A good tip is to concentrate on one colour, if you want to have control over the black squares, trade your white bishop for a knight!
This is mostly nonsense.
Is controlling the centre only important in the opening?
No. In general central control is very important in every stage of the game.