Actually, the use of the gray card was centered more on exposure consistency across a multitude of lighting situations more than anything having to do with film processing:

Gray card - Wikipedia, the free encyclopedia

In more than 30 years of commercial (still) photograpy, I have worked with a couple AD's who also wanted the card included in the first frame of film simply as a "control" or reference color patch so later Photoshop corrections could be based on the color of the gray card in the lighting setup we were using. Using the gray card as a control for later changes in "processing time..." was never called for with any of my clients.

I shot the EX3 today in Colorado in the shade on snow with a 20000k white balance... pictures looked great! Usually in Queenstown New Zealand in a similar set up I would hit 10000k to 12000k... As long as it looks good I don't mind what the numbers say.