The point of the ending wasn't to show if he was in a dream or not, but to show that he didn't care, since he left the top sitting there and went to his kids. Notice he had never left it alone, the whole movie. The top was taken differently by many people. Some thought that, since it sorta wobbled before the screen went black, it stopped spinning after the movie ended.
There are also many theories as to what happened. I believe in one of them. If you remember back when they were introducing the sedatives, Cobb was put under with the sedative that was supposed to keep him out for 3 hours. However, he woke up right after. Then, he got up, and ran into a bathroom, where he spun the top. The top proceeded to fall off the sink before he could verify if he was in a dream or not. The next time he spins the top, is the end of the movie.