It depends (you're right, Kevin!
) upon the music style, the instruments, the vocals, and the song arrangement.
As far as "general guidelines" - for which there are always exceptions - here are a few to get started:
- Because of the omnidirectionality of bass frequencies, instruments residing solely or mostly in the bass frequencies (e.g. bass guitar, kick drum, Brad Garrett) tend to be grouped near the center of the mix because panning them has little effect long range and sounds less natural short range.
- Except in specialty genres like the many isotopes of the heavier metals and some space synth effects, hard panning is usually reserved for room ambience. In metallurgy, the opposite is often true.
- A nicely balanced mix often means keeping instruments that can easily clash in fundamental frequency ranges seperated from each other. For example, if you have two rhythm guitars (electric or acoustic, doesn't matter) that are playing independant rhythm tracks (they are not doubling lines), it's usually best to keep them seperated by at least a a couple dozen degrees in the pan space. The same can be true for a guitar and a piano.
- In general, a good balance left-to-right is accompanied by a good balance in frequency spread. In other words, try to spread the frequencies evenly L/R. A bass-heavy L and a treble-heavy R is usually not so good.
- A symmetry between lead vocal and backup vocals is usually desired. This can be something like lead vocals centered and backup vocals in a symmetrical L/R stereo spread, or it could be lead vocal mid-left and backup vocals centered on mid-right, or any variation thereof. But an imbalance in vocals (all vocals mostly left or mostly right) is usually not as good.
- If there is only one vocal, pan center unless it is, in effect, "dueting", "doubling", or otherwise sharing the spotlight with a main instrument such as lead piano or lead guitar. In that case a symmetrical balance between the vocal and the lead instrument can be in order, much as if the lead instument were a set of backgorund singers in a call/response arrangement.
- When setting up your soundstage, think three dimensionally: think not only left-to-right, but front-to back, and low freq-to-high freq as well. Keep a balance in all three of those "dimensions" and you'll find plenty of room to fit everything.
HTH,
G.