Jill, the fluctuations day-to-day are exactly why I weigh daily instead of once a week!
I have this incredible fear that if I weighed once a week, I would just happen to weigh on the day that my weight fluctuates up a couple pounds, and I would think I had gained even if I really didn't. I think as long as you understand the fluctuations, there is nothing wrong with weighing daily. I usually don't count a weight as "official" until the scale has reliably read it for three days in a row.
I do make sure I weigh at the same time every day though -- first thing in the morning, after using the bathroom, putting in my contact lenses, and brushing my teeth!