In the shops I've worked in, mostly the windows clients are playing back proxy. But then again, I'm in Apple country.
Not sure about other places but due to the FCP integration I see a lot of shops that start on Macs at hires and then hand off proxy to producers on PC's for logging and tagging. That's a pretty common workflow due to the sheer numbers of FCP editing systems out there.
I'd love to hear from some Win shops too as to what their experiences have been dealing with hires footage in CatDV on Win boxes.