I weigh almost daily (nekkid, first thing in the morning) because I dont care about fluctuations. Actually I have gotten to the point where it is exciting to me to say...HEY the high has been below X for a whole week too! Although I only count lows.
I am amazed at how much difference just a couple hours makes. My normal wake up time when I run is 5 am. But if I can sleep in I will. The difference between weighing at 5 am and weighing at 8:30 am is almost always over a pound (less later)
My usual pattern over the course of a week is low on thursday-saturday adn then sunday or monday my weight climbs up and then it takes tuesday and wednesday to come back down to the previous weeks weight and then try for a new low. Related to hard workout days and light workout days.