If I ever get done with this fracking project I'm on right now I'll try and do some tests before I go home. Generally, I try to use the GI always, but drop it's quality down to -1.5 for outdoor scenes, unless you start noticing blotches of shadows on your buildings/objects, then you'll need to bump it back up, but it definately makes a huge improvement as far as realism is concerned and I've had a ton of people comment on how much better even the simplest renders look with the GI enabled versus standard lighting.