I should qualify that and say that based on the rank tracking I've been doing, so far every time I've manually checked my results they've been spot on but, as you point out, Google's own documentation suggests that this may not always be the case.
What I was getting at, but wasn't very clear about, is that these results are far from instant due to the fact that you're looking at potentially 27+ API requests per search term. (3 search engine APIs, up to 9 pages of results for each -- Google's API maxes out at 72 results)
I currently do this every hour while in testing and store the results in my datastore. Then I in the web-app I pass this data to the Google charts API. The result is presently something that looks like this - http://imgur.com/PidBG.png