I believe it really depends on the type of equipment you have and how the individual inputs get their signal to the amplifying stage. In my particular preamp case, each input RCA goes back to a hard-wired input selector switch and is out of the actual circuit unless it's the one selected. I suppose the wiring to all including the used input could be subject to stray RF (or other input analog signals) and that in some way is getting into the amplifying circuit, particularly if they might be bundled together back to the selector switch. I worked on a friend's preamp who's inputs were bundled together back to the selector switch and interestingly, there were two inputs that were connected to two separate devices, (tuner and CD player). When the CD player was selected but not playing, you could turn up the preamp volume control and hear the tuner playing and vice-versa. Unbundling/separating the input wires and the sound from the other went away. Also interestingly, it didn't matter which input was used with which devices as the cross talk was evident in all of the used ones. So there was definitely cross talk between inputs happening. I didn't even think of using shorting plugs on the unused input RCA's at the time, however I'm not sure they would have made a difference. If you use them and notice a difference in improved sound, then it's all good and I believe each preamp will have its own input isolation or cross talk levels, including stray RF. In my opinion, it's in the topology (and possibly the electronics) of how the input signals get to the amplifying stage.
