Parents can choose to sign OUT of special ed at any time. The same cannot be said for signing INTO special ed.
It is true that refusing to sign an initial IEP leaves the child in general education, with no accommodations or services beyond what is available to the general population. I believe OP is in a re-evaluation situation, which is somewhat different.
If parents don't sign annual IEPs, the child remains under whatever conditions the previous (now-expired) IEP had them on, under stay-put provisions. For re-evaluations (triennials), depending on the state, failure to sign a consent to re-evaluate may exit them from special ed, especially if the child is not enrolled in the public schools at the time. If the child is enrolled in public school, parents can actually reject testing, and not have the child be exited from special ed, or the school system and the parent can mutually agree to waive testing for this triennial cycle, and write the IEP based on other existing information.
However, I actually think the most important consideration in the case of the OP is accommodations for post-secondary education and the standardized tests requested when applying to post-secondary institutions. From this distance, it's hard to predict what your children will need at that point (which is only about five or six years distant, at most, if I recall their ages correctly--maybe less if they end up further grade accelerated). Both post-secondary gate-keeping tests and post-secondary institutions want a history of accommodations that dates back more than a couple of years, and will want testing that is no more than three (sometimes two) years old at the time of application for accommodations. If you maintain continuity through IEPs or 504s from now until then, it will be an easier sell at the post-secondary entry level.
Plus, some districts will actually pay for/provide related services to homeschoolers, especially those who have previously been enrolled in the system. Again, you can always refuse services, but it may be that circumstances will arise that make accepting services beneficial to your family. And you can't accept what hasn't been offered, or be offered anything without up-to-date eligibility documentation.
Caveat: Of course, you all know that I'm a little predisposed toward testing...
