When do presidents assert executive privilege? When it's in their interest
Nixon's is the most famous case. He refused to turn over documents (in this case the Watergate tapes) until a unanimous supreme court made him. But he didn't have to resist. In the next major imbroglio the president chose to defer to congress. Gerald Ford marched up to capitol hill and testified in person about his pardon of Nixon. Invoking executive privilege apparently never occurred to him, which was fortunate. After Nixon the last thing the public wanted to hear was that another president was refusing to come clean.
The next big scandal was of course Iran-Contra. Once again Congress wanted to know what the president knew and when he knew it. Reagan decided to give them everything they wanted.
Clinton was the next president to face a major scandal. He chose to fight congressional demands for documents and testimony.
In each case there's one simple theme. Self-interest.
If President Bush wants to assert executive privilege, let him go ahead. But we shouldn't be under the impression he has to for the sake of his office. The Decider can decide the issue without worrying that he is under any obligation to assert a privilege on behalf of his successors.