My understanding is that your camera attempts to expose the mid range of your photo to be 18% grey so these card are most useful for exposure. However an incident light meter will be a bit more effective.
Depending on your camera model you should be able to measure a scene and set a custom white balance based off off a reference photo. This reference photo can be of any neutral coloured thing from white to dark gray, or even better a colour chart, and does not need to be even in focus, so I doubt you specifically need a grey card for this purpose, however it would be useful.
I think this would probably be the most effective way to set the white balance, although given that you could have multiple light sources at a different temperature even this is not fool proof.
To set a custom white balance on my D600 I use
PRE setting for white balance. For more information: