I have a child that scored way up there on the NNAT as well. We chose not to do additional IQ testing. Because we homeschool, we've had the opportunity to do open ended achievement testing for a much more reasonable amount of money. That has been very worth while over the last several years (found out he was GT at 6, he is now 9). As your child gets older, talent search tests are also interesting and can provide valuable data for not too much money.
I think that is a personal question. I think it's FINE not to do further testing. Even if you have more IQ data, schools might not always care or accept it. Chances are, if they aren't jumping over your daughter's NNAT scores, they aren't going to jump at a similar score on another IQ test. Teachers used to comment on my son's NNAT score - it was a standout in a group of many MG kids. One teacher even told us we should find another school or homeschool based on it. No teacher offered us additional accommodations because of it!
However, if you have open questions or seeing some inconsistencies across your child's abilities, further testing may be valuable to you. Good luck!
