If you were using a css reset it would reset your styles in all browsers and would help ensure a better chance that your design would look the same in all browsers. A good reset also would remove the borders on many/most elements. I never do a project with you a css reset.
A CSS resets the defaults on as many styles as possible that were browser specific and helps increase the chances it will look the same in other browsers. Browsers many times will add something to an element by default and others won't. We end up designing the site in a particular browser and then when we check another browser it doesn't look the same because of that extra styling on an element that was added by that browser.
Adding a reset to a site that has already been designed could possibly mess something up because it was not started with the reset. If you add the reset on an existing site that didn't have it before, go through the css and the site and make sure everything is looking okay.