Recently, Brad in WI asked the following:
Does anyone use several grey cards in a room to get accurate color balance? There are plenty of tutorials on how to use a single grey card but they frequently have it placed near a window or tungsten light. Frequently, it's near the edge of the frame which can influence color. Even twisting it one way or another can alter the white balance. If several were placed in a room at various locations, each could be on a separate layer and blended or painted out to ensure correct color in all areas of a room. Even an Expodisc varies depending on where it is placed in a room.
I'm sure we will get many viewpoints on WB (white balance) but I recommend you don't need to get too worked up about it. I've never used a gray card. I just set my 5DMKII or 5DMKIV to Auto WB, shoot RAW, and use a manual off-camera flash. I find that I rarely have to adjust WB in post-processing. Usually, the WB is right on. The worst case is a little adjustment of the Temperature slider in Lightroom. As Dom Bower describes above, "It's not about accurate WB, it's about good WB." That is, what looks good is more important than what's accurate.
Probably the most important part is to use at least one flash, even if you are shooting brackets. The flash helps get the whites white and the colors accurate.