When shooting RAW the in camera WB only affects the preview JPEG that is embedded in the file. It has no effect on the actual RAW data. The WB is only set when you output from the RAW converter. WB in RAW is therefore a purely PP choice. If I think that WB might be difficult due to mixed lighting I might shoot a grey card for later reference, but at the point of exposure not something to be unduly worried about. The other thing to remember is that the choice of picture style can affect the Histogram shown on the camera. The camera uses the histogram from the JPEG preview conversion, if you have your camera set up to produce previews with a "strong" colour cast this could adversely affect your histogram, and thus your exposure choices.
If you want your histograms to accurately reflect the data captured then you can use a Unity WB that shows that data correctly, although this shows a strong green cast on the preview. Otherwise it is probably best to set the neutral camera style, with minimum contrast and saturation. In natural light I would then use Auto WB as it is fairly close. With artificial light though I would set WB to the Tungsten or Fluorescent pre-sets as appropriate, as that is when Canon's AWB seems to fall down. This will show you a reasonable interpretation of the data captured by the sensor.
Alan