I have been thinking old skool, when I used to shoot everything with a 1/4 or 1/8 promist filter. I used to use this technique all the time shooting Beta SP and Digi, but it kind of, went out of fashion. The wider the lenses got, shooting with permanent filtering became a pain. If you are Ob doc with a wide lens, you pull wide and get the edge of the Matte Box in shot.
But it does look good for interviews.
So I have my bag of filters and my Matte Box which I can now mount with my 5d, and I did a little test. The results are subtle but I think worth it.