For me, I normally use the auto white balance settings on my camera... most times, it's pretty much spot on. But if I'm doing a shoot, I'll leave it on auto, but always shoot an 18% grey card as well with every lighting change. I shoot in raw so that in post, I can make any adjustments that I need to with the shot with the grey card in it. Having said that, sometimes that doesn't even look quite right and may only use it as a starting point.