Top Posting FIXED.
You say "rule," but shouldn't you mean exception? Are "most" browsers
maximized? Of those that are, what about 1024x768 monitors? You get all
this empty space with them! Or worse, the user gets a horizontal
scrollbar, and is _forced_ to maximize his/her window to remove it.
What if a maximized, 800x600 user has their "Explorer Bar" open, looking
at history or favorites?
How does a website fixed at a width of 768 pixels do when accessed by a
user with a 640x480 resolution? Or how about by a portable device?
The simple fact is, the only time a 768 pixel-wide design is convenient
for the user is in _one_special_case_. And that is with a maximized,
800x600 browser window, with the explorer bar closed, with the Office
toolbar off, and with the start menu at the top or bottom. And that is
pretty uncommon.
Because a flexible design serves _virtually_every_ user, I don't see why
fixed-width designs are considered at all.