It is not ever that simple as it very much depends on the song, arrangement, what and how many tracks/instruments there are and how they are panned, what effects are being use and how are they panned, and dozens of other little nuances that need to be understood. The answer is that you try every possible combination until you find the one that sounds best for the song. Some of those combos are are gonna be much better than others, narrow it down from there. Keep in my mind that what sounds real cool and can come across as nice sounding ear candy, may also be too distracting or not fit the emotion and message of the song. You should know it when you hear it if you always focus on being true to the nature of the song and make that top priority.

I had a whole dissertation in mind to this but after AG's perfect answer theres nothing more to say!

I will add that I personally am of the school of Sir George M. as far as panning of vocals go.I want multiple tracks of the same or similar takes so I can manipulate them in a basically mono L/R format.If I have a basic two-voice vocal section w/backing harmonies,I will put the Two-voice section hard left together and hard right together and spread the harmonys throughout in a three dimensional spread as support.