My reply to this is just a repeat to what the others
have already said. But just to clarify with samples from
my own work.

Here is how I represent the part symbol in my Orcad
schematic component library:



There is no need to force there to be an actual copper
connection between the common pins on the PC board because
it already exists inside the switch. However, as shown
above it is good to show this connection in the schematic.

Now, as Peter has indicated these connections can be useful
in making a matrix on a one sided PC board. (This also can
translate to making all the connections in one circuit layer
if surface mounted versions of the switches are used). There
is no advantage using these internal connections for a matrix
if through hole versions of the switches are used on boards
of two or more layers.

Here is a schematic idea for how you draw the matrix if the
internal connections are used:



Michael Karas