I weigh in every day. This used to be a problem for me, and I would get discouraged when I would fluctuate up half a pound. But I track my food on a spreadsheet (neurotic much?

) and like to keep my morning weight there too. It's just nice to see the downward trend, and I can also keep better track of how my diet/exercise affects my gain/loss. I've recently started looking at my sodium, and can tell by the spreadsheet that when I have a lot of sodium, my weight fluctuates up almost a pound the next day.
I think it's okay to weigh every day, but only if you can tell yourself that your weight WILL fluctuate from time to time, and can keep from being discouraged by this.