RS232 Grounding question
You need to wire the signal ground (Pin 5 on DB-9 RS232) always. 

Using only TX and RX is simply not a valid wiring IMHO. If it works, it's by accident and you are relying on the having the exact same ground potential on both sides. Which is ok for an internal UART connection within a device, but not between two separate devices. 

If this causes a ground loops between two devices, or unwanted potential shifts, using fully opto-isolated RS232 as mentioned earlier is the only correct way.