I'll just get out of the way the detail that this is what I do professionally, and then give you my general thoughts, and our personal anecdote.
If you have a referral question(s) (something specific that you need to understand about your child), then formal evaluation can be extremely helpful, particularly (perhaps only) if you have a skilled and nuanced clinician. If you need it for access to supports, again, it can open many doors. Otherwise, I tend to be conservative about testing just to have a number (without interpretation and specific recommendations).
For our family, we have not pursued formal testing for our children, but then, we homeschool, and I'm an assessment professional--so that might be as much of an argument for as against testing in general. We don't need it for advocacy or access purposes, and I trust my own clinical skills enough not to need someone else's confirmatory scores.
BTW, the CogAT is usually scored using age norms, so it shouldn't make much difference which form was administered. But based on it being before the start of first grade, it was probably the CogAT7 form 5/6.
