When the source you are recording can be considered reasonably as coming from a single point, then you might as well record these in mono. Examples of point sources are vocals, bass, sax etc.
Some sound sources are 'wide', for example drum kits, and these are best recorded in stereo.
Some 'wide' sources, e.g pianos, harps, congas and acoustic guitars, can be recorded either in mono or stereo, depending on their place and importance in a song. For example, if I am recording a singer with just a guitar, I most likely will record the guitar in stereo, the vocals in mono. However, if the guitar is a background part of a bigger arrangement, then there is little to be gained by recording in stereo.