I would think it would go something like this.
If you weigh yourself first thing in the morning after "evacuating" lol anything you can get rid of bathroom wise then you will get the most accurate reading of what you actually weigh.
If you weigh yourself every night you will have the extra weight of any food or water still in you. If you are consistant about the time (and don't have an unusually large meal) you will be able to gauge your pounds lost or gained. You just won't know exactly how much you weigh because it will always be just a little inflated.
That's why I do it first think in the morning, buck naked after the potty. I even take off my glasses. I don't want to see one extra ounce!