Never leave the storyline dangling. You should have tied up any loose ends and answered all the editor's questions to prove that the potential readers will come away content. I don't want to give you the impression that everything in the outline is carved in stone. Plotting isn't finished until the final revision pleases the editor. I've changed many a scene, devised many a new twist, even come up with a more powerful ending in the middle of writing a book.
Writing is an ongoing process, a natural one and so naturally, it's flexible.
The farther you get into your story, the more attuned you are to the characters and the more insight you have into their motivations. A direction you mapped out in your outline may no longer be the best route. Constantly check for consistency and forward momentum. You have to let your instinct as a writer guide you, but also have the common sense to evaluate any major detours before you get yourself lost. A detailed outline can save you from countless hours of blank screen-itus and from an uneven and meandering plot.