Some trainers I know tell their clients to weigh once a week. Some advise their clients to forget the scale altogether. They claim the tell-tell sign should be how cloths fit. I tell my clients that have weight issues to weigh everyday first thing in the morning. There is always discussion about the bathroom scale not matching the one at the gym or the doctor's office. The important issue is that the numbers are moving in the right direction.
My reasoning is profound if I do say so myself. If weight lose is an issue and especially if it has been for a while, once a week is not enough to maintain a focus. Weighing everyday really gives that phrase "in your face" a different and acceptable meaning. A lot of information can be stored and forgotten from Monday morning to Monday morning, but not from Monday morning to Tuesday morning.
It might be grounds for a temporary depression (hopefully without a trip to the refrigerator) or it might be motivation to make a change.