In thinking about whether to test, I would ask what you hope to gain, and whether the cost will be worth it.
Is your child working 2+ years ahead in multiple subjects, or 3+ in a single subject? Are there other reasons to think that she would score in DYS ranges on the WISC? If she does score high enough on the WISC, do you plan to have her achievement tested, or do you have a compelling portfolio to support the potential IQ test results? Perhaps most importantly, why do you want to gain entry to DYS-- what do you see as necessary or helpful benefits of the program for your homeschooled child? Will you still be just as happy with your child if she doesn't score at the 99.9th percentile for entry to DYS-- that is, can you divorce your own wishes from this process? Will you still feel that you've gotten value from the testing if she doesn't qualify?
(I think the answer to that last question depends on whether you have reason to believe that your daughter may have learning disabilities. I think it's most important to teach a child at the proper level and pace, but results of IQ tests and achievement tests like the Woodcock Johnson don't really help with that, instead just telling you how rare a particular set of test results are. This info won't help you much in homeschooling your daughter.)
There's IQ testing and IQ testing. Be prepared to pay more for testing with a professional who knows about gifted children and their quirks, rather than an intern at the local college or a garden-variety psychologist who sidelines in IQ testing for money.
Aside from any decision to test, I would begin by assessing carefully what leads you to question whether you've been underestimating your daughter. Is she ready for more advanced material or a faster pace than you've been giving her? Does she have needs or talents that aren't well satisfied currently? Does she ask for more or different work?
