as with all creative decisions there is no right or wrong, just choice
it is also important to remember that being in stereo does not mean that there is a signal in both channels, but that the signal can be far left or far right - that is, a sound can be anywhere in the stereo spread and still be defined as being stereo, even if the source is mono - it is the placement within the stereo Left-Right spread, that gives a sound it's own place within the wider mix.
In addition to simple panning, techniques such as Haas-Delays (delaying a source by a short amount (2ms-45ms) in the opposite channel) will give the semblance of that source being more in one channel or the other...