I weigh myself Monday - Saturday right after running. Clearly it is the water weight that changes the most, and doing this kind of helped me figure out what the effects of different things I eat/drink/do are on water retention - amount of salt I eat, alcohol, etc.

The advice of "Weigh yourself as little as possible" kind of makes sense in light of the fact that a lot of information can get confusing, and it's only useful if your "experiment" is controlled to some degree, so to speak. In other words, if you're eating the same food everyday and do the same exercise everyday, then it may make sense to take daily measurements to figure out the effects of something else, but otherwise - fuggetaboutit.