The old saying that you can't shoot what you can't see has a lot of truth to it. I try to shoot the tiniest groups that I can and do quite a lot of paper punching. In my case, 12X is kind of my minumum as my eyes aren't as good as the used to be. I just bought a sightron 8-32 for a rifle that I am having made. I'm feeling that 32X is going to be too much, but we'll see when everything comes together.
I used to be able to shoot vey well with 9X, but I'm finding that as years go by, they seem to be making targets smaller and smaller and not as clear as they used to make them. I'm also finding that Leupold is not making enough threads on their eyepieces anymore to get the adjustment needed.
Your magnification needs are certainly dependent upon what type of shooting you do as far as range, what you are shooting at, and what your needs are as far as field of view.