When you do this, check in all browsers. I might be doing something wrong every time, but whatever widths I set in HTML get screwy in each browser. What seems to be the right width in FF and Opera is wider (or thinner, I can't remember anymore) in IE, and Konqueror just can't get anything right-- often the width is so wide it goes off the screen, or at least outside the container. Damn that browser all to hell, acid test my butt.
Setting different widths in the CSS seems to make a difference, esp since I noticed someone (I can't remember who it was again) seemed to ignore the CSS so I actually did finally end up with most browsers looking alright (except Konq of course but fortunately nobody seems to use that one).
Someone might know why I see this, and it might also have to do with my using Gnome as my desktop setup, as it's definitely screwed with Firefox (form inputs especially, they are much too long when viewed in FF on any Linux distribution I can find who's running Gnome... KDE seems ok).