With the most common "normal" zoom lenses the minimum focussing distance at 18mm may not be the same as the minimum focusing distance at 55mm. So just focusing and then zooming in may actually produce an out of focus photo.
p.s. The higher the f/ number setting the more depth of field you will get in your photo. Therefore on closeups you should sacrifice shutter speed (use a slower shutter speed) to allow for a higher lens f/ number so that more of your subject will be in focus.