It depends. I always use separate mono tracks for the left and right of a stereo source, that way I have complete control and can pan and process both channels independently. However, not all synths or samples are actually stereo. If they aren't, there is no use using a stereo track for it, or even two mono tracks for that matter.
Anything that is captured with one mic, like vocals, is mono and just needs one mono track.
However, if you know that you are going to use a particular sound in full stereo (meaning that the left and right channel are panned wide), you could use a stereo track just for the convenience of it.