As I recall the tradition is that the senior White House correspondent is given an early question, without fail, and then at the indication of the end, holds the tradition of thanking the President on behalf of the assembled press corp. It was that way when she became senior correspondent many years ago.
Now the contention will be that GWB has snubbed the WH corp in general by his treatment of their honorary senior. The response, of course, is that neither Helen Thomas or the rest of the corp of WH reporters appears to take the stature of that unofficial position very seriously based upon the tone, content and editorializing of her "commentary" from the floor. If her collegues couldn't corral her conduct, someone needed to before she resorted to outright dramatics during a news conference.
This was a big deal in the tradition laden WH.