Is it important to correct the White Balance on a bracketed series before I process the set of photos in an HDR program like NIK HDR Effects or Photomatix?
Yes, it helps to avoid unusual color shifts in the HDR (High Dynamic Range) process particularly in the blue skies. I think of HDR as an amplification process and within that process you get more saturation, noise and details. Also depending on the color space you use in the HDR software colors can shift as the color space is changed from sRBG to Adobe RGB to Pro Photo.
When I process a bracketed series of photos for HDR, in Lightroom I start by selecting the series of photos. I first look at the white balance of the normal or middle exposure of the bracketed series to check the white balance. Most of the time my camera is right on or very close to the correct white balance, but occasionally it’s off and I need to correct it. I will start by comparing the Auto adjustment to the As Shot to see if there is a big change in the photo. Sunsets can be problematic as the foreground in shadow can be much cooler than the warm light in the sky.
In the example photo of the Taos Gorge, the camera As Shot white balance is 4900 for the Temp and -1 for the Tint. Lightroom’s Auto white balance setting is 5750 for the Temp and +11 for the Tint, much warmer that what the camera setting produced. As you can see there is quite a bit of difference in the two photos and at the time I took the photo, in May, the sage brush and grasses were light green and covered with new growth, so the As Shot white balance is correct for the foreground vegetation, but the blue sky has a slight cyan cast. The Auto white balance processed photo has a lot more haze and I found it impossible recover the greens in the foreground without introducing a yellow cast in other parts of the photo. In the final image, I adjusted the blue channel to remove the cyan in the upper sky, darkened the blue channel and added a little saturation.