What I think will happen, is you will have to restart. When you start battling the elite four, you will notice that they insert intervals between each member, that allows you to recharge and more. Use this time to save. But, when you reach the champion, you will notice that you can't stop and save like normal. You would have to wait until the end of the pictures, conversations, credits, etc., until you see the save icon. Therefore, if you take your game off while the champion is talking, there is a high chance that you would have to battle him again, or whenever the last time you saved. The only thing I can tell you is, WAIT! Don't try to rush it. Same thing might happen if your game freezes or shuts off. If it doesn't make it passed the save point, OH WELL.
When you reach passed to the home screen, whether you take it off or leave it, the same effect will occur. Nothing good or extra will happen.
Hope I helped! :)