One A- instead of an A or instead of a B+ makes no real difference. Of course, there is a bigger difference if you say ALL your grades are A- instead of ALL your grades being A.
I am in a top 10 school and I have several B grades. In fact, I have B grades in exactly the topics I am pursuing graduate education. But those grades didn't matter because the majority of my grades were A- and above, and that I have plenty of research experience in the topics I applied to grad school for.
Grades/GPA are just one of many ways the graduate program will evaluate your application. They are looking for evidence that you can succeed in their PhD program and produce a lot of papers/results. Grades show that you have the potential to do really well, but many other things (actual research experience, strong LORs) demonstrate that you can actually do what they want you to do.
In my opinion, once you pass some minimum** that does not raise any flags, GPA/individual grades are one of the less important parts of your application. Because applications are judged holistically, you will see many people with 4.0s being rejected at schools. But if you look at these profiles, you may see other major problems with their applications (no research experience, weak LORs, didn't pick the right schools to apply to, lack of research fit, terrible interpersonal skills/attitude etc.). Sometimes, it's not evident from the profile posted (since not all information is provided).
(**I think this minimum is something like 3.0, 3.3, and 3.5, depending on the level of competitiveness of the school in question).