Certainly a top that spins forever is an indicator you're dreaming, as its impossible in our physical reality. But within a dream, anything is possible, including tops that fall or don't fall.
The spinning totem is fine for proving to yourself that you're in a dream, but it's useless to prove that you're not. In fact, there's never any way to prove you're not dreaming. There's nothing you can say or do right now to convince anyone, including yourself, that you're certain you're not dreaming. As a fine example, some of you will read this, think about it, and, in fact, dream about it and in your dream, you'll hear yourself saying "I know I'm not dreaming!"
So, that being said... doesn't that negate an awful lot of a very necessary premise of the movie?