A few things I would recommend considering -
1. Will it change anything? Will testing help access gifted programming or modifications? If so, check that the school allows outside test results and if gifted programming doesn't start until later check the rules on the age of test results (ie - if the gifted program starts in grade 5 and testing needs to be less than 2 years old then waiting could avoid retesting). Also, note that you can't retest for a couple years with the same test.
2. Do you suspect there are other labels that might apply? Twice-exceptional (2E) kids are extra fun
Do any of your children seem to be struggling with writing, reading, focus, etc. In some cases, 2E kid's exceptionalities sort of act to cancel each other out and they might seem rather average.
3. The child's personality/temperament. Testing an uncooperative or extremely shy kid makes it challenging to get an accurate result. For most kids, maturity decreases the issue and they might test more accurately in a year or two.
4. $$ - let's face it, testing isn't cheap. We were lucky to have some coverage but there were yearly limits. We tested our kids in different coverage years to make the most of our benefits.
My experience which I base most of the above on -
School was a disaster with DS from the start. By October of grade 1 the school urged us to test him for ADHD. We didn't think that applied but agreed there was something going on and that testing would be useful. We tested at 6.5, DS was not very cooperative (throwing things, hiding under the table, etc). I honestly thought the tester was going to laugh us out of the place for thinking he might be gifted. Turns out he tested gifted and likely LD (written expression). He was too young to be sure about the LD but it was enough to get some change at the school.
In grade 3 his teacher recommended doing a second round of testing because he needed an official LD diagnosis to qualify for AT (a school laptop) and more accommodations. We tested again. At 8.75 DS was MUCH more cooperative and tested much higher. We kind of suspected that the first set of scores was an underestimate but didn't realize just how far off they were.
We tested DD at age 7 and I feel that we got accurate enough results. She was much more cooperative and isn't as freaked out by new people and experiences as DS.
Both kids are now in full-time gifted programs which require testing to access but that have all sorts of rules about acceptable tests and strict time limits. Many families in our area end up having to retest because results are deemed too old.
Good luck with whatever you decide.