Well, I would never end a book in the middle of a battle or so just for the sake of having a cliffhanger. I know that the first book has to have some sort of build up towards an exciting/interesting ending, even though it is intended that the main characters real quest is not yet over. Would you say that Eragon is an example of a first book in a series that would work well alone, or is in not enough of a completed story arc? It does leave a lot of unanswered questions for a single book, yet the ending does finish the immidiate threats etc.
Both can be standalones I guess. What happens in between the beginning and the ending is what counts though. A book might build up to a death, and end with it, or end with them deciding that they will travel. What it depends on, however, is how many questions and subplots there are which are not answered. How much more the reader would expect to be told, I guess.