Kind of a broad, high-level answer, but I hope this helps...
A mixer/sound designer up here at VFS (Brad Hillman) likes to use the E.A.T. acronym for background edits - Entertaining, Accurate and Technical.
I've found that helpful. So to bastardize his intentions, here's my interpretation of that:
Entertaining backgrounds are, in your example, those sorts of whispers, low drones, radically pitch-shifted elements that speak to an emotion or sense of eerieness without being an identifiable object in the scene, but nevertheless clue the audience in to what the story is saying somehow. Or, they are the comically distant tire squeal or bit of walla that manage to poke through when there's a lull in the dialogue. They are the parts of the background that get a tiny bit of space to speak without stealing the listener's focus, just subtly supporting the scene.
Accurate being, cut the precise right version of the thing you're seeing. If you're outside and it's raining, don't just grab any rain. How close is the camera perspective? How dense is the rain? Is it slappy and fat, with just the occasional spatters? Just kind of misting? Is it a torrential downpour? Is the rain hitting puddles, cars, etc.? Same thing with winds. If you can't see a visual reference of what the wind's doing, you might not want to cut in excessively howling winds. If you're cutting in a ringing phone, what era are you in? Don't give me an iPhone ring if I'm watching Madmen, etc.
Technical... I think of this as, choosing sounds on the basis of how that sound would be heard in the reality of the scene, and not just because it's a good-sounding background. So if you cut in a residential neighborhood dog bark, don't be afraid to filter that a little bit and try to give it a bit of distance. A mixer on a condensed schedule probably doesn't have time to rolloff and pre-send verb to properly place every single element you throw at them, and you will save them some time by taking them halfway there with tastefully baked-in effects in these cases. Outside traffic shouldn't have a bunch of really crisp highs on it. Rain, if you're indoors, should sound like it's hitting something (a roof) instead of gushing down in front of you.
In terms of speaker placement...
- make sure the stuff you are panning center is there to support the production audio (e.g. cover up the production noise), and not pull attention away from the dialogue. If you can get a guide track before you start cutting ambiences, that will help you out a ton.
- Don't be afraid to hard-pan things left and right.
- And be careful about putting basically anything with a transient attack, or that would be noticed, into the surround speakers.. unless the scene calls for something like that.
Backgrounds are really, really tough and require a ton of imagination and careful selection. I find them incredibly difficult and creatively challenging, even moreso than hard FX sometimes.
Remember that like a lot of cinema sound, their purpose is to serve the goal of the story and scene rather than to purely sound cool. Unless the scene's focus IS on the backgrounds of course!
There was an amazing interview with Tim Nielsen on Designing Sound re: backgrounds a month or two ago - http://designingsound.org/2011/08/tim-nielsen-special-interview-on-backgrounds/. Some good stuff in there.