David PH is right, and the Nikon guy was wrong. There is no difference between setting correct white balance in-camera and adjusting the Raw in post-processing. The camera applies correction at the JPEG stage, not Raw, same as in post. The only thing I would say is I like to get the colour approximately right in-camera because it gives you a more accurate LCD image that is easier to assess when working. I prefer to use the camera's pre-sets for that (daylight, tungsten etc) rather than rely on auto-white-balance. Some cameras are pretty good at AWB, other less so, and I can usually guess it near enough. The other thing about AWB is that it can change slightly if you're shooting a lot of similar images under the same light, when they should be the same, eg different groups of people at an event. Having fixed WB fixed with the pre-sets makes things easier in post-processing because when you've corrected colour for one image, it can be applied to all the others.
How WB affects auto exposure is a different question, and it's possible (in theory) that it might shift slightly in evaluative/matrix mode. The latest cameras have very sophisticated metering systems, with RGB sensors linked to focusing and subject (face recognition etc). Details of manufacturer's algorithms are very complex and not published (and they're not necessarily accurate, either) but if you have WB set approximately right then that's working for you anyway.
As for shooting Raw vs JPEG, if you read the forums it's easy to come away with the idea that you can do anything in Raw, but next to nothing to a JPEG. While the former is true, you can actually do quite a lot to a JPEG in terms of colour balance or exposure etc and not notice any difference. However, if you post-process anyway, shooting JPEG is pretty pointless - you lose the benefits of Raw (well worth having) and you also lose at least a stop off the highlights straight off the bat by the in-camera process. That seems like madness to me. Raw uses a lot more memory, but that is cheap these days, very cheap.