If she was in a dream, she could have conjured another top in her pocket (or "found" it there if she was still thinking it was reality).
And why did he lock the top away? Did he know it was all a dream and was trying to hide it? There's really no reason to hide the top unless he knew she was right after all...
Subconsciously, her magic top would NEVER stop spinning, so she would always think that she was in a dream whether or not that was true.
Only if inception were a form of mind control that could alter a person's perception of reality and override their normal senses. I never got the sense that it was presented that way. And if that's what inception means, then objective reality has become meaningless. Dom could have had inception planted in his own brain by someone else (I was waiting for Arthur to be revealed as some kind of nefarious guy), and not be aware of it, and that's the end of him being able to tell reality from dreams.
Otherwise, the whole movie is just about a guy taking a nap, and none of it matters.
Maybe that is the point: that it doesn't matter. In fact, that's the interpretation that allows the ending to be something other than a silly joke. It's not "we didn't show you the end because we couldn't figure out a real ending to this movie," but rather, "we didn't show you the end because the characters can't tell the difference anyway."
So even if the top fell over, it could still be a dream. And in fact, if you're going to end the movie with the top "wobbling," then why no go all the way and show it falling over? If you're not going to show it falling over, why have it start to wobble? I think it's because it didn't matter either way.