% Have there benefits, but I generally dont use them because it can be user unfriendly, especially when you present readable text. Your sentenses get to long for a user to read comfortably.
There is also an other point of view in this discussion. Its not only about 'make your site fit on every screen' but its also about design. When your making a design, you deside yourself how wide your page will be. You can design it 100px wide, but it might aswell be 1200px wide. It's your call!
For a user friendly side which should be read by a big group of people (800x600, 1024x768, 1200x1024, 1280x800, IE6, IE7, Firefox, etc) i try to pick a width between the 800 and the 900px.
Good luck designing and constructing your side.
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