I like the advice given in this article from Hoagies:
http://www.hoagiesgifted.org/why_test.htm

We have tested our son twice. First time he was 5 years 2 months old. We just tested again at the age of 7 years 5 months. Both times we tested to help us answer questions about his educational programming. We needed answers because things weren't working. He was unhappy, hated school, acting out at home sometimes, etc. We had a sense that there was something we needed to do but didn't know what. We deliberated a long time before testing the second time. It was worth it because the testing answered our questions and gave us new direction for our son.

I wouldn't hesitate to test again, if more questions come up. I think we'll have our son test with Midwest Talent Search at Northwestern for the sake of achievement testing and keeping a handle on just where he is in terms of educational needs. But not likely to ever do another IQ test. I don't see how it would be needed.

Read through the Hoagies article. See if it helps you to decide. As Grin says, why test if you're happy with how things are? But if you need answers, testing is a good place to start.