Howdy,
Revisiting the NGS page http://www.ngs.noaa.gov/web/news/GEOID12_Error_Notice.shtml I see that there is now a problem area west of Dallas.
Extract below is from the page linked above.
GEOID12 should not be used in the areas shown in the map. In most of the areas shown, the errors are less than 10 cm. However, larger errors are suspected near the center of the large areas west of Dallas, TX and in Alexandria, LA (as much as about 40 cm), and south of Meridian, MS (as much as about 15 cm). At present no other areas of GEOID12 are known to be affected by these problems, and we believe GEOID12 can be reliably used elsewhere. Please note that this error has no affect on the current gravimetric geoid model, USGG2012.
NGS apologizes for any inconvenience caused by these errors and is working diligently to identify and fix the problem as quickly as possible. Analysis of GEOID12 will continue and we will provide additional updates if other problem areas are identified. We expect to release an updated version of the hybrid geoid model no later than the week of August 13, 2012.
I noticed where I live in West Houston there appears to be an area of uncertainty with the Geoid12. I know there are subsidence issues here and one USGS publication I found directly mentions the West Houston/Katy area. Estimates from this publication put it at around 0.10'-0.12'/year, I believe I read.
Hopefully this .pdf link works
edit: I don't know that the subsidence issue is relevant.