We tested at 4 and once more after that. The results were very useful to me because, not only did I have numbers that were useful to me, it also showed me my son's strengths and weaknesses. For the last 2 years, I have increased the rigor and acceleration of subjects that my son likes to study to a far greater extent than I would have normally done if I had just assumed that my son was bright or gifted. The level of giftedness convinced me that he needed more acceleration than was possible in conventional settings.
I have also used his test results to get him into some gifted programs for enrichment and meeting peers. And he has 2 mentors who are reputed in their fields who teach him and guide him - and the reason they accepted him was based on his IQ test results, his achievement test results and his work portfolio in that subject.
So, if testing will gain you entry into select programs or help you find peers for your kids, I would recommend you to do it. Testing also helps to identify any issues like vision tracking, learning issues etc and if you suspect any thing of that sort, then it is good to evaluate earlier than later.
Good luck.
