The camera's WB doesn't affect RAW images, so that's possible only if the camera outputs JEPG whose WB processing algorithm is "better" than that of computer processing software.
But I read somwhere that cameras collect data for different channels RGB and than applies WB. Too technical for me so maybe I explained it wrong. However, some say that even though one can play with WB in post it is not as precise as one would hit it the very first time.
My idea of using expodisc was a bit different than suggested. I'd like to use it in a room with multiple sources so I woud just point at the scene not each source and help camere make a good balance. Does this make sense?
Sense it makes, it may also be good enough, but it's not going to properly fix the color balance. That's because color is not given by the object, but by the light that hits the object and is reflected by it. I know that people are taught that objects have color, but that's simply wrong. Light has color and objects reflect a part of the light spectrum, with various intensities. For instance, if you have what you think is a purely red object that's illuminated by purely blue light, you'll see the object appear black, that is, it absorbs the light. So what is the correct color of the object? The one you think it is.
In the case of multiple light sources, you get a varying mix of seriously weird colors (coming from light bulbs).
Details here: http://www.cambridgeincolour.com/tutorials/white-balance.htm
Also see there white the green-magenta correct is available:
white balance uses a second variable in addition to color temperature: the green-magenta shift. Adjusting the green-magenta shift is often unnecessary under ordinary daylight, however fluorescent and other artificial lighting may require significant green-magenta adjustments to the WB.
What is not clear from that phrase is that the correction is necessary because the color balance was affected more in those areas of the spectrum, so that needs to be compensated separately than the rest of the color balance. This still doesn't describe all the differences in color from what a human expects, but it makes things good enough.
Also note that since the light spectrum reflected by the objects is not what the camera sensor was designed for (which is likely to be D65), you'll loose a lot of tonal definition, so color corrections in those areas of the spectrum will produce seriously noisy results.
Things like white discs may be used with good results if the light is consistent, so will not need to change the WB for each shot. But if that white disc will produce photos with white walls and shirts but people with orange or green skin, you should know that it nobody's fault - it's just the light.
This article is very good in explaining practically white / color balance and its perception by humans: http://www.ianplant.com/photo-tips-how-to-white-balance.htm