I have read the Apple white paper and other people's articles about this and there's one thing I am not clear on. My souped up 2011 IMac won't play the 5k R3D's (although the white paper suggests that it would) so clearly I have to transcode. The choice is between optimized media (PR4444) or lower quality proxies. It seems that I can start editing with the optimized option straight away, but I have to wait for the proxy media to finish. So why bother with proxy at all? The high quality Pro Res files play just fine and are better quality.
I will be grading (with Davinci) and conforming from the original R3D's btw.