In a dream, the top never wobbles.
In the last scene, last 3 seconds or so the top develops a significant wobble.
However the main point the movie conveys is more complex
Cobb, like us, started with his mind aware at level 0.
Cobb, with the help of technology, knowingly goes into knowingly fictional realities (the dreams level 1+)
Cobb has a top that he knows how it acts at level 0, this is how we know at the end he is not at level 1+
However there still is Mal's theory that there is a level -1.
We do not know if there is a level -1. The top only verifies that Cobb is in level 0.
So at the end, Cobb starts his routine check that he is in level 0.
He changes his mind and decides that the reality of a level is not an important detail.