I think it's probably been said already but I thought I should weigh in regardless ..
Firstly I love Inception, it's what really got me into lucid dreaming in the first place, so don't see this as bashing the movie or anything

I just believe that a totem such as the spinning top wouldn't be completely reliable, it all relies on your own perception and expectations just as most things in dreams do. The whole idea in the film was that they were completely unique to the person so that they knew they weren't in someone else's head and therefore it couldn't be replicated the way they remembered it. I can see this working as a check but only if you truly expected it to topple in a dream and gave it enough time and strongly believed you were already dreaming. If you did it as a casual passing check as if you were still awake it would still behave as such, as if you were awake and would topple simply due to you thinking you're awake and your expectation of it saying it should topple.