I would have to say, finding a balance is imperative. In that sense, as a moderator one should take action, however one should also consider the consequences. A warning may sometimes suffice, or in other cases banning, or even in other cases calling in the auxiliary. It's not say that positive action shouldn't be taken, however. But the form of positive action may vary to have greater effect.
"So it seems the best way to keep your mod job is not to take extensive positive action." [Asq] I agree with statement to some extent. Extensive positive action is only taken when it's called upon. And it's always your call.
"The key to being a successful mod is to make executive decisions that are not harsh, unjust or unreasonable, but reasonable in the context of the situation" [Minai] I don't think it necessary that decisions are not harsh, unjust or unreasonable, so long as they accomplish what needs to be done - removing spam, sending a warning (not necessarily a PM), etc.