I've never seen any on my images.
I just checked mine anyway by mounting a standard filter and looking at the corners on the ground glass with the lens stopped all the way down, then with it wide open and saw no evidence of vignetting. I then looked through the camera from the corners of the film plane with the lens open (of course) and saw no difference with the filter on or off. I next tried with two standard filters stacked and saw nothing on the ground glass, but looking through from the corners of the film plane could see a bit of the second filter.
I think even a screw-in polarizer would still be thin enough.