I disconnect it when not in use just to be on the safe side, some electronic devices drain power slightly even though it is switch off. If the lipo pack is already very low in charge but still connected to the transmitter, slight power drain over time might cause it to overdischarge and vent inside the battery compartment.

I know it is a hassle to unplug it at the end of the day every single time but I still think it is a cheap insurance to do so. If one worries frequent plug and unplug might damage the connector on the transmitter, one can use an male-female extension wire that is plug into the transmitter, when the connectors are worn out then just get another extension wire.