I saw some circuits, in which the component's GND pin's are connected to the Centre Tap of the transformer. But no wires from the board lead to GND.

in the following "gnd" is the common in your circuit "ground" is actal ground

as far as the actual circuit, a lot of confusion arise from the fact that gnd is not necessarily ground.

For instance my systems which are installed in Public transport have a 'gnd'. However we do not require the bus to drag a wire around connected to ground :).

So, as far as your circuit consider ground "0V", "Reference", "Basis" or whatever.

Now, in many cases is is an advantage to connect gnd to ground since that can improve noise rejection in many cases, an example would be cable shielding.

Erik