I do weigh daily, but

a) I'm a member of WW, so it's kind of at the front of my mind right now (currently half way to my goal weight) and

b) because my weight fluctuates (like shootingstart, up to 3lbs per day), I like to see the whole week's worth of weights so I can get a better picture of how much I've lost over the week. If I only weigh once per week, I might weigh on a "heavy" day and feel more disappointed than I should.

I also take tape measurements once a week, since that seems to give a more accurate picture of what's going on that just weight alone.