I don't think it's wrong to weigh yourself every day though. As you say hydration can change your weight, so surely having 7 recordings in a week will give you a better idea of the overall average than one, because that one day could be an anomaly and give you a false idea of whether you're going up or down.
Also my weekend readings are usually higher because I'm more likely to have been out drinking and eating late at night so if I'd only been seeing them I wouldn't know that during the week I'm often half a kg lighter.
I don't think it's wrong to weigh yourself every day though. As you say hydration can change your weight, so surely having 7 recordings in a week will give you a better idea of the overall average than one, because that one day could be an anomaly and give you a false idea of whether you're going up or down.
Also my weekend readings are usually higher because I'm more likely to have been out drinking and eating late at night so if I'd only been seeing them I wouldn't know that during the week I'm often half a kg lighter.