Hi Alex –
To my knowledge (others feel free to correct) the high quality button is there to enable showing the full res version of a DV codec QT movie. by default, a lower resolution of DV is displayed for playback purposes, but the HQ button is there to allow you to over ride that. I typically use it if i need to do a screen grab of a DV clip for documentation of something else. This should no effect the quality at all when laying back to tape or encoding into another codec.