If you are concerned about your site being usable to your readers then setting absolute font sizes is not a good idea. This means that if one of your customers has their browser set to a larger font size than you consider normal, they won't be able to read your Web page.
If you set your font sizes to percentages, this will insure that your pages are readable and look good no matter how your readers have their settings. 100% is the default size that the browser is set to, less than 100% would be smaller and more than 100% would be larger.
This is especially important if you design with a PC and a lot of your customers use Macintoshes. Setting your fonts to percentages means that the pages won't come out as unreadable gibberish on the Macintosh.
