just sharing my experiences (after having been beaten up by BB, and he was right doing so).

rendering GC: should be ON, and 2.2.
if you don't do that your image will suffer from severe and non-recoverable dynamic loss in the dark regions. BB showed this in a scene with a white ball reflecting on a just 2% reflective black box. With no CG you cannot see reflections, even not when pumping up the darks in post (Photoshop), even not when using 32-bit HDR image formats.

material GC: should be "like rendering" (or 2.2 manually) for all color textures
and should be 1 for alle value-textures like bump, transparency, etc. There is a script doing this for you.
If you don't do that on color, than a full-red texture (external source) will deviate from a full-red colorswatch (internal). If you don't do that on values, you will loose contrast in the mid-gray (20-80%) region, showing less bump, less specular, less transparency except for the extremes.

When these settings make weird results, check the materials setup. For instance, the Vicky-4 HighRes textures from the package throw in a cyanic color swatch disturbing the diffuse channel. Fine for the default colored lighting but a nightmare when lighting is set to plain white, and wrecking your GC interpretations. When I set the color swatch and the lights to white, all was fine.

IDL: is great for radiosity and self-illumination effects, and for large-scale / outdoor scenes using a SkyDome object. But do note that you need explicit lighting to get the shadows and highlights in, like photographers are using flash when working outdoors.

Explicit light setups on the other hand are preferrable for indoor close-up portraying studio results, as it will be the shadowing which does the job. Buth then, like photographers are using white, silver and gold panels, IDL can be of support as well.

All the best.

- - - - - 

Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though