Ceres, to make sure we're all on common ground, this only applies if you have a scale in your bathroom.
And really any kind of weighing yourself will work as long as it's at around the same time each time you do it and under roughly the same conditions.
For example. If you weigh yourself when you first wake up nekkid and after going to the bathroom you're going to be much lighter than any other time of day. Imagine hopping on the scale the next evening and thinking you've gained five pounds through food or water or just 'day'.
Or if you weigh yourself at the gym with your gym outfit on and shoes off every time you're there it shouldn't be too much of a difference.
The important thing is consistency. Where you think your weight is in those two scenarios is a little different. For example I might be 176 at home in the morning, and 179 at the gym. But if I'm consistent with both measurements I'll still see the same amount go down as I'm losing weight.
When it comes to water weight. Imagine eating a bag of chips and some Chinese food. The next morning you might hop on the scale and freak out because you think you've gained X amount of lbs. Chances are most of it is water weight and you'll lose it throughout the week.
Sodium, salt, makes us retain water. I don't remember the biology of it, however it's true. And our diets have more sodium in them than they should anyway, so when we do things like eat mr. noodles or other such things it is quite shocking to our systems.
It's important to look at your weight over time. As long as it's going down and not up over each month you're good! This is a really cool site, check it out. Here's mine. : P
http://weightchart.com/member.aspx?u=Seraphsody