The answer depends on the screen resolution used by your site's visitors.
Screen resolution is no small Web design issue.
If you design your Web site for resolutions higher than 800x600, many of your visitors will have to scroll horizontally to view parts of the site. One school of thought is to design dynamic pages that adjust to the visitor's screen size. This is done by defining the width of HTML tables used for page layout in percentage of viewable area. However, this gives less control over the position of page elements within the table. As a result many Web designers prefer to define their table width in pixels.