I'd guess that, unless you're doing fast turnaround episodic TV stuff, it's best not to prefabricate 5.0 backgrounds. That way you can have more control in the mix.

I asked a similar question a while back, and got some awesome answers:

http://socialsounddesign.com/questions/2889/mixing-backgrounds-atmos-for-5-1

I'd guess that, unless you're doing fast turnaround episodic TV stuff, it's best not to prefabricate 5.0 backgrounds. That way you can have more control in the mix.

I asked a similar question a while back, and got some awesome answers:

Mixing backgrounds/atmos for 5.1