I just looked at the configuration options for the sound devices in Parallels, and there is no way from within Parallels to make the default OSX sound device "exclusive" to Windows.
However...
I also tested with my handy-dandy little USB sound device, and with that, I was able to make it "really exclusive". When you plug in any USB device, Parallels gives you the option to assign it to the "Mac side" or to the "Windows side" (and to switch back and forth as needed). I assigned my USB audio device to Windows, and then it installed and showed up like a "real" sound device (instead of routed through the Parallels virtual sound card driver).
Exclusive mode on that was really exclusive mode. The device didn't even show up in the OSX Sound System Preference panel, and OSX couldn't play any sounds to it.
So, my guess is that your best bet would be to use an external USB sound device and assign it to Windows if you need bit-perfect playback under Parallels. Aside from that, we'd have to really just test it to see what the normal virtual driver is doing. If they implemented it smartly, it might still be bit-perfect if nothing is being actively "mixed" from the OSX side, but you'd have to plug in something that can record into the digital output and test it to be sure.
