Increasing detail above 0.5 is often necessary. What I recommend is not going above 1.0, except in specific circumstances where it may be necessary. Even 0.75 is a notable improvement over 0.5 and still has less render time than 1.0. 0.9 can provide virtually identical render quality to 1.0 and again with lesser render time.
If lighting is your concern, changing detail isn't going to give you a lot of control. The GI settings aren't there to make your scene *lighter* - in fact they may often make it darker due to higher accuracy. If you want your terrain to be lighter simply increase "Strength on surfaces" in the Enviro Light settings. Likewise with the atmosphere. That will be the best way to lighten or darken your atmosphere or terrain separately when using GI.
As far as GI accuracy goes I usually use between 1 and 4, never any higher than 6. 2 is usually a safe bet. I also seldom use GI Surface Details, but it can be useful in situations with complex foreground detail at a small scale. In other cases it just increases render time needlessly. I would try without it first to see if I liked the results and only turn it on if experimentation with other settings (and using crop render to test) showed that there was no other way to achieve the result I wanted.
In general be sparing with the detail settings. You usually don't need to set them as high as you might think, and certainly using detail as a way to affect something like scene brightness is the wrong way to go.
- Oshyan
