Nash, to me your generalization is just that. Also, it again reminds me that you don't appear to know as much about US history as you think you do.
We've had some totally bad Presidents. IMO, it's unlikely that Bush will ever be perceived as the worst, given some of his predecessors.
Beyond that, he had some pretty fine moments after 9/11 that are going to mitigate against "worst".
Further, it's WAY to early to really see Iraq in what will become it's true perspective. If it really does jumpstart democracy in the Middle East, Bush will certainly not fit the "worst" category. Au contraire, he'll probably get the "visionary" label like Nixon got for "opening up China".
To sum up....... I don't think it's wise of you to try to write history before it happens. At least don't chisel out the text in stone yet.