As an assessment professional, in general, I am a testing hawk (except, of course, when I am swamped at work
). I think it can only be helpful to understand your child (and for them to understand themselves) better in any way. Cognitive & achievement testing are one aspect of that understanding.
OTOH, we have not chosen to have any of our own children tested. It has not been a gatekeeper to accessing any services or resources that might have been of value to us or them, so far (but we homeschool, so formal GT programs have not been relevant). If it became evident that testing had practical value (say for documenting accommodations on high-stakes college entrance exams), then I think we would consider testing.
I am also, of course, in the low-incidence position of being able to informally assess my own children with some degree of confidence. I suspect I would take a different position on testing for our children if I were not.