Forgot to mention that the reason I actually discovered this issue was when I was doing 12mm spacers all around. At first I was trying to run 12mm ECS spacers up front and they would not sit flush, I had another set of 12mm that I was running in the back which I switched to the front and they fit just fine. So the 12mm ECS spacers I had did not fit the front.
Currently on my setup I'm running 5mm front and 10mm rear 42DraftDesigns spacers with the 66.56/57.1 conversion so I don't have to run hub rings on my 66.56mm hub wheels. They fit perfectly and I highly recommend 42DD spacers.
This is really interesting. I didn't realize there was a spacer that can take the place of having a centering ring. I just got some OZ wheels and they have hub centric rings that seem to be pretty good and not create vibration but I was thinking of slapping some spacers on the rear. One of my worries was how a typical hubcentric spacer would take to being mounted with a non 57.1 center bore wheel that has a centering ring installed.