I'm doign it ever 2 days when I also get my bodyfat checked. We use this info to determine any meal changes.
Normally I'd agree w/ doing it no more than 1/week because it is really sort of incomplete data as far as gauging progress if you dont' know your bodyfat. It can start to mess with your mind if check it every day & then start obsessing about it.
But in my mind it depends on what you use the data for. If it has a purpose, then keep that as your context, otherwise you can freak out over it. The only place its even relevant for competition is for bodybuildign if you are trying to get into a particular weight class. Figure only uses height as a class identifier.