For connectors there are extra pins (for mounting?) e.g.: 19 & 20 in this picture

When I go from schematic to pcb the net-checker will point out that they are disconnected. On headers I use the 'x' as in

Is that the correct approach? When I go to the PCB view, it will ask then to remove that net, which I guess is the point here. Will the part still be properly assembled?

Or should those always go to GND?

Thanks!