I've messed around with this. In Windows, I've preferred 9300 because even with the brightness way up, 6500+/- seems dim. Higher color temp equals more defined graphics, which is what you want in a high res monitor. For landscapes and stuff, yeah, lower color temp makes it look rich. I used to love 'copper' and the like lensed glasses.
I just tried 6550 (that's #2 in my monitor's line-up, #3 being 'user') with Raiden and Twin Cobra.....and perhaps they are more accurate. I don't know as I like them better.
Incidentally, for those who prefer a higher temp in their OS use, I think adjusting the default MAME options could give you a lower-looking temp. Of course, lots of people are going whole hog with HLSL, and adjusting saturation, color drives, etc. (Sort of the point of the feature, ultimately.)