You really have it "bassackwards"
IMO, the question you should ask is how do you want to shoot your HDR. Once you decide what works for you, then you can standardize it to one or more of the 'C' modes.
The simplest, shotgun approach is to use Av mode, evaluative metering and set to 3, 5, 7 shots, someplace between 1 to 2 stops apart until histograms shows all tones well in the images.
A better way of doing HDR will depend on how far you can or want to push the highlights and where you want to place the highlights in the bracketing sequence. Much of this is dependent on your camera and post processing software and workflow. This will also require some testing to know the relationship of your camera settings/histogram to real clipping points in your software.
I have done some testing, so have a method hat works for me....but, it is continually being evaluated and adjusted as needed. One C is set to M, spot metering, multishot, bracketing -/0/+. Based on the scene, I will set the number of shots and EC spread.
Then, while doing AF, I will determine the brightest (significant) highlight and place it (+EC) where testing has shown should place it ~95-96% in LR 4. A quick evaluation of the dark tones will confirm my guess on initial shots/EC settings.
Frankly, using this method will, on many scenes allow you to maximize the dynamic range enough that the HDR shots are mere insurance.
John
