It is really going to be a personal decision - and depends on what you hope to find out from testing.
Note - if your state is as rigid as mine is about the age cutoff, test results won't sway them for early entrance.
We actually met with the psychologist when DS was 3.5, without intending to test, because we had started to question our original thoughts about DS and public schools. DS was in daycare - and DS's personality and development made us realize that we may have to re-think about what we should do about kindergarten. He missed the state cutoff by a day or two, and our area is notorious for being strict with that age cutoff. We saw "flashes" of giftedness, but DS is one of those kids that blends in so well that most people look at him and see a very average boy (even the psychologist was surprised by the results because DS is so low-key, and nearly fooled him at first) - and we were concerned that in public schools, he would be one of the kids completely ignored because he does not make noise and focuses on blending in.
We ended up agreeing to testing because even though it can be unpredictable at his age, we were also looking for any answers that may help us figure out what to do. For us, it really opened our eyes - his results completely stunned us, and the recommendations given were completely the opposite of what we thought originally would work that we really had to re-evaluate our plans for his early school years.
Note - the test for 3.5 is not going to give as many answers as you think. I am glad we did it because before it, we had viewed DS as being very bright and maybe MG, but the results showed he may be beyond MG - and that he was hiding some of that from his teachers (we had no idea what a normal 3 year old would know or be interested in, so he just seemed like every other 3 year old especially since he was a late talker). We will have to get him tested again sometime in the next year or two for a better picture (and his school requires it), especially since our family history has a lot of 2E history, and we want to make sure we catch any potential LDs earlier than later. Cost for us is not as big a concern as it is for some families and I know we are lucky (especially since my parents are in complete support of what we are doing) - so if you are looking to push for early entrance to public schools, I don't think I would recommend it.
But - it did make us aware we needed to make changes immediately in his daycare and ammunition to push for those changes (early promotion up into pre-K and then pushed for K 4 months later at DS's request - he started to show early signs of boredom and acting up), and look at long-term school environments for him.
We realized we could not wait until he was basically 6 to start kindergarten, or have him in a situation where he does kindergarten at his daycare and then repeat at private school (which many parents with kids missing the cutoff do in our area). And we started looking at private schools where age cutoff was not a limitation, and where he could be happy - we found a GT school. He has started at our local GT school, and has gone from fighting to stay home and 10PM bedtimes to hurrying out to the car to get to school, and having a normal bedtime - and finally finding a peer group that he wants to be with (and where they are not in his face about his young age or size).
So - for us, it gave us the information that we did not have, and the ability to act on that with confidence that it is what DS needs. All our actions to date and the happiness of our DS today has validated the test results. But it does not always work that way with this age - so please take this with a grain of salt - we are one of those families where it did really work out.
