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.
