i weigh daily. if you do that you have to accept that you will have lots of days where you stay the same or gain, and not let that upset you. easier said than done, perhaps.
for me, the problems with weekly weigh-ins are these: if i wait all week for a weigh in, and then it's rubbish, that's a horrible, horrible shock. i found it really upsetting. depressed the hell out of me. The second reason is that some days your weight blips upwards, because of water or whatever. If i only weighed in once a week, and that happened to be a day where i was retaining water, i might think that was the full story - and it would get me down for the whole week.