Until recently, I have mostly used the presets of sunny, cloudy, etc. I got into this habit back with my D200. The problem normally was one of changing light conditions. Over the last few months I have been using auto-WB in the changing conditions and been quite happy with it. However, auto-WB does have a limited range to it; from Thom Hogan's D700 book:
Nikon claims that this function works at any color temperature between 3500K and 8000K. Note that most indoor lighting falls below that range! Moreover, my experience tell me that the D700 gets less accurate towards the extremes. I'd say the most accurate range is much narrower, perhaps 4000 to 6200K.
Almost of the time, I have something white in the frame. While not necessarily planned that way, I am pretty sure that this helps with the accuracy of the camera's WB guessing.
---------+---------+---------+---------+---------+ Joseph K Seattle, WA, USA