90% would be a fluid width and not a fixed width. I normally set fixed width at 980px but now that Brogan mentioned 992px I might have to give that a try.
I like fluid width to some extent but when you have a monitor larger then 20" it gets hard to follow text paragraphs and it also makes laying out articles with pictures difficult to work on all monitors and resolutions.
I actually had a hatred for fixed widths but I thought i would develop a fixed width derived from mjps thread and they really aren't that bad. I guess the last time I used it I had to compensate for those 800x600 users that most probably put me off fixed solutions.