Ultimately, only you can answer this question. I'll share my experience in case it goes you insight.
We sent Snoopy to kindergarten knowing he was at least highly gifted and knowing that his teacher didn't get it. Halfway through the year, he was behaving weird, depressed, not himself. We had him tested and found he was PG. The tester did an initial evaluation and concluded that PG was a definite possibility, so he used WISC even though Snoopy wasn't quite 6. Snoopy still hit ceilings on several test parts. It was critical that we have that testing done then and it worked well for us.
DD, however, thrilled to playing dumb. When she was 5, we still couldn't figure out whether she was as smart as brother - or smarter (just trickier) - or just a normal kid. We had to have her tested. Cost big bucks and all we got from it was "yes, she is smarter than the average bear" - but were told the number was nonsense because of the lack of cooperation.
We had her tested again at 8 and she still didn't cooperate fully. The psychologist said that he would have sworn that she was doing her best, but when he reviewed the notes of the prior tester, he noted that she got things right 2 years earlier than she got wrong with him. Hence, she was still faking it.
So, what did we learn? Test when you have to test, but don't expect your child to act like someone else during the test. From what you say, there is a good chance you may not get an accurate reading from the test scores.
Think about this - what would you do differently if you got an IQ score of 140? 150? 160? Granted, with the right tester, you get a lot more than a score.... but what will you do differently? Would individual achievement tests be a more valuable tool at this point? Knowing where to start with the homeschooling? It may even be easier to get DD to cooperate with that kind of test, depending on her personality.
You might try asking homeschoolers whether they have found this type of testing useful to them.
Good luck
Mary
