Of course this is a minor thing, I don't think anybody is caring so much about an imprecise tag, however I'd like to know if there's a general rule to follow in order to understand when it's appropriate to tag, for example, a C++ question with the C tag.
The tagging is generally used to indicate to which category the question is related to. There are no general rules but basic common sense on the part of OP while asking question.
If someone feels wrong tags have been added to question, then people who have high reputation(>500 I think) can use retag facility to give proper tags. If people do not have reputation to retag then they can just leave comments for others to notice and retag.