Okay first revelation #1: with a 50mm lens on my 50D... it's literally the same focal length as my eye. If I have both eyes open, right eye looking through the camera, it feels like normal vision.
Now with that said, so far I've had my diopter adjusted ALL the way to the left (the - side) because that made the AF boxes/text readout the clearest/sharpest.
But then I just tried an experiment where I left both my eyes open, lined up an images so they look like one image to my eyes as I would when I normally look at things, and the right image, through the camera, was totally blurry. I then closed my left eye and realized my right eye has to adjust everytime to make things clear through the viewfinder.
So I suppose it would be better to adjust it so things are in focus when my eye is relaxed right?