Custom white balance looks bad/off when set on the 5d Mark iii

[lulugrace]

I got my Mark iii in June and every time I set a custom white balance it doesn't look right. I have used both a purchased grey card and a white piece of paper, both look bad. When I set it on my Rebel Xs it looks perfect, no adjustments needed in post processing, but not the case with the Mark iii. Shouldn't it be the other way around, considering the price tag on the iii ?

[lulugrace]

I'm not sure what a white balance shift set is. Are you referring to in camera?

I know it doesn't take too much time when I'm editing studio portraits. But when I have 7 client sessions to edit in a weekend any extra step is bothersome. And I just really didn't expect to have this issue with such a high end camera. Also, if I'm shooting outdoors just for fun on a whim and I don't have my WB card and nothing is neutral in the photo, it make it a guessing game of hand selecting the WB in post.

Do you have this camera and find you have the same problems?