In writing, just as in reading, we are instructed to start at the beginning and work forward to the end. However, there is a situation in writing when you might want to consentrate on the ending before completing previous chapters. The point in time you most likely would need this tactic is when the plot is stuck in a rut, you can't think how to push forward no matter how you try, but you're close, so very close. Skip to the last chapter. If you've done your planning with an outline and know where you want your story to end up, then write it. Allowing that scene to come out, making it come alive with all its description, dialog, and action, might very well help you finish. Once you have written it, work backwards to develop and write those previous events.
I've always felt that outlines are an organized prewrite actitivity. However, its rigid structure can be and should be tweeked here and there as you are writing your story. Of course, there is the chance that it will back you into a corner, and your creative ingenuity won't help you escape. (And here you thought outlines, all that beautiful planning, were supposed to keep that from happening!) Welcome to the writer's world where characters often decide to rebel and go where they want the story to take them. Just remember, no matter how far estray you may go and find yourself in that corner, there are tricks of the trade to get back on track. And finally, my best piece of advice is to KEEP WRITING! That's a must. It will all work out in the .... ending!