I think it is really happening, both because that ending is what makes more sense in the story and character development and I felt the wobbling was pretty apparent.
I think in addition to leaving some doubt, the reason to not have it clear was a bit of a joke on Nolan's part. Cobb's team talks about the need for catharsis and reconciliation and so Nolan sets up with the top wobbling where the audience expects to get the moment of reconciliation and then decides to cut it at the instant before the top falls to deny that moment.