There's a definite tension between the rule of not accusing other users of being shills and the reality that there are quite a few shills out there. I think it a still good rule, but not because it's never right. Rather, the rule is good because the false accusations do more harm than letting some shilling slip by.
There is more that could be said here, but, really, if you take this as your default approach to analysing things you read on the internet, you'll be headed in a good direction the vast majority of the time. It's not a completely black and white thing; for instance one certainly should hold out a healthy level of skepticism if, say, the message and the speaker seem to be completely incongruous, but mostly, let ideas stand on their own.