stereo or mono tracking?

You should only record stereo sources in stereo; everything else should be mono.  Vocals, guitars, classical instruments, single drums, etc. are all generally recorded in mono.  Stereo sources can be the overhead or "room" drumset recording, stereo keyboard with left and right outputs, and maybe something like a choir could be recorded in stereo as well.  Most of my recordings are mono.  SOMETIMES I'll do acoustic guitar in stereo. I'll point one mic toward the fretboard and one toward the sound hole.  This can create some interesting tones, especially in a solo. Otherwise, if I'm just doing some strumming of guitar, I'll just do mono.

It should be noted, however, that you can mix mono sources different in the stereo spectrum.  For example, you may have rhythm guitar more toward the right and lead guitar more toward the left in your final mixdown.  Both are mono sources, but you can move them around in the stereo mix. These are not rules... just possible options.  The mix formula on every song is usually different... unless every song has exactly the same 'lead' instrumentation.