Mental Ray Rendering

All of these settings can be accessed by opening up the Rendering Dialogue, under the Rendering Tab. In this new Dialogue box click "Adjust Exposure..." near the bottom. Please note that you must be in a 3D or Camera view for the Rendering Dialogue to be available.

Intense Early Morning
Exposure Value: 13
Highlights: 0.6
Mid Tones: 0.3
Shadows: 1.5
White Point: 7500
Saturation: 0.7

White Point has been increased to produce a more 'warm morning light' look. Mid Tones darkened quite drastically, Saturation decreased to compensate. This setting may be a little too intense depending on the contents of your model. You may want to increase the Mid Tones and decrease Saturation to your liking.

Evening
Exposure Value: 14
Highlights: 1
Mid Tones: 0.5
Shadows: 3
White Point: 7000
Saturation: 1.2

The key here is decreased White Point, when compared to the morning, to give a 'blue' light (I find the default 6500 too 'blue'), and darker exposure. Mid Tones are also quite low.

Daytime
Exposure Value: 12.5
Highlights: 0.065
Mid Tones: 0.5
Shadows: 0.2
White Point: 7500
Saturation: 1.8

This is a good general daylight setting, that is not washed-out or too dull. The main disadvantage (mainly due to the bright exposure) is the blown-out highlights, for example on a shiny, metal roof.

All-in-all I feel that most perfectionists will still want to 'Photoshop' their renders. These settings will hopefully provide a better baseline image to work with when compared to the default settings that, I feel, produce a washed-out, dull image.