Wiring a stereo TRS socket as a mono output
Whether the jack plug gets shorted or not depends on the geometry of the insulation on the plug and the width of the contacts on the socket. Assume the worst - whatever that may be in your design.







simulate this circuit &ndash; Schematic created using CircuitLab

You are free to wire the socket any way you desire. 

a) This will work with a mono or stereo jack wired tip and sleeve. A stereo plug will have the ring floating and may cause hum in that channel (usually the right).

b) Will work similarly and has the advantage that if a stereo lead is plugged in that the channel will be grounded rather than left floating and producing a hum.

c) This won't work if a stereo lead is plugged in as there is no ground connection.

It's your choice.