I have to put a plug in here for using a tester who specializes in gifted kids if you do have a need for an IQ test, because we got much more than numbers out of our SB-V assessment of our then 4yo. The reason we tested that early was because we wanted to know if we should send DS to school a year early. The psychologist we saw analyzed DH's and my personality types along with DS's scores (DS was too young for his own personality test), and was able to give us information about potential difficulties we might have in various educational settings and also how we as parents might deal with that. The whole testing process was quite helpful. And the psychologist did tell us of the drops in scores over time, and that helped us feel a little less freaked out! She also had the experience of having seen similar score profiles, and she told us some examples of different school situations that worked for kids with similar profiles.
In addition, the IQ scores got us in the door early with the school, who then did their own achievement testing. So, even though I agree with other posters who say it's only one piece of the puzzle, to us it was a useful piece that led to what is currently a pretty good school situation.