As Vincent said, as long as the HTPC is putting out 1920x1080, you're good.
Video cards have overscan compensating resolutions (like 1176x688, etc) and those are last resort if there is no 1:1 mode. Those resolution should not be used otherwise. I have no problem using overscan/underscan scaling compensation as there is no degradation in quality.
It sounded like the THX mode forces 1:1 which should be the default mode anyways.
I view exclusively OTA and always use ScreenFit (1:1 mode) on my Samsungs. The channels or programs that have noisy borders and not common. For the 1080i channels, the picture is noticeably softer when it is on the overscanning 16:9 mode. Both Samsungs of mine overscan 720p signals. I wish there is a ScreenFit mode for 720p channels like ABC and Fox.