Selector Switch switch, output should be left open or grounded?
You have a DPST (double pole single throw) switch, and want to use it as a SPST switch.

The obvious answer is to simply ignore one of the poles.  This is what your bottom diagram shows.  The circuit shown by the top diagram would also work, but is just asking for trouble if something shorts to the top left contact.

You absolutely don't want to connect that top left contact to ground. Think about it.  When the left switch closes, the 24 V supply would be shorted to ground.  Bad.

Of course a even simpler answer is to use a SPST switch in the first place.