Consistancy is indeed the key. I prefer to weigh myself first thing in the morning, after I drain myself out, lol. i call it my empty weight, and its the lowest it will be in the 24 hour period. however, like said above, it is the average over time that we really care about. saying you were 150 pounds for a few hours one morning doesnt mean much if over 2 weeks you generally weighed 155-158. Then again, the number itself isn't the most important thing anyways either.
the other way to look at it is to consider what is the real world number. since you are going to eat and drink as the day goes on, and most of your activities will be done later on in the day, then your weight at those points in time are your true working weights. for example, a wrestler weighs himself at 152 in the morning, but comes back at 5pm and weighs 157. Now he has jumped an entire weight class, and that is his working weight, not the 152 he weighed when he was empty in the morning. he has to wrestle in that 157 weight class and at a body weight of 157.
but really, whats in a number? it is really only the change in the number that matters, not the absolute low or anything. weigh yourself at the same time of day, and you will get a fairly accurate picture of how your weight has changed, and that is really what is important.