what the video is played back on and what you edit with.
If you think it sounds awesome, then continuing to use it can't hurt - and you can always turn it on or off in the menu as you see fit.
If you are playing back on gear that is not 5.1, then using the extra space is unnecessary.
If you want the audio behind the camcorder to be recorded, then turn it on.
If you edit the video and the editor can't deal with importing or exporting 5.1, then what happens to the other audio tracks?
I am wondering what the best sound option is on this cam? I have recorded with 5.1, and it sounds awesome, but I dont need surround. So the question is, will plain stereo be as good as 5.1?