Since: Dec 04, 2007
I don't know how accurate or realistic you plan the orchestral tracks to sound (i.e. full orchestra, or just the strings sections, etc.)
But, what I would do, is look at a seating chart for an orchestra. At the very minimum this will inform you of how a typical orchestra is laid out in the sound stage.
The separation of strings will create a natural panning, as violins are typically panned a bit left with a fairly wide sound, cellos a little to the right with a narrower sound, violas also to the right, kinda mixed in with the cellos, but maybe a little narrower, and contrabass further to the right, and somewhat narrow.
It's kinda hard to describe with words, so:
Choir will be separated kinda similarly.
Typically, you might have Sopranos a bit left, tenors a little less left, bass a little to the right, and altos a little further to the right.
At least that was how our choir was laid out in the practice room. Though sometimes we'd go mixed choir, and disperse our members somewhat randomly.
And, of course, there are variations of the seating depending on the conductor's preference and the size of the ensemble.
You probably don't want to drown the orchestral/choir sounds in reverb, but you might want them to have a little bit more than the guitars/bass/vox/drums to set them further back in the sound stage.
Strings also have a fairly strong presence in the mid and upper frequencies, as they call all play up into the treble clef pretty well, and there is some overlap in the instruments themselves in their ranges. So, you might want to scoop them a little bit EQ-wise to make room for the guitar/vocals.
I don't know if that's the "right" way to do it, and you'll have to experiment, but that should get you on the right track.