Using a CSS reset means you, in many cases, will have many rules that are not necessary. You will first set body { margin: 0; padding: 0; }
in reset.css and then set body { margin: 0 auto; padding: 20px; }
in style.css or something, when only specifying the latter would give the same result. And will you really use all those obscure elements that are included in the reset?
What I sometimes do is that I add a reset.css, start coding, and then I check my page in Dragonfly or Firebug and there I will see which rules from reset.css are overridden. Those I can remove from reset.css, and in the end the file is likely to be very small or empty.