I weigh every Saturday, I don't do it at a specific time each week, no reason why... probably just laziness. But I plan on starting to do it at the same time each week if I can (morning). At the moment I weigh in just my bra, on an empty stomach and after going for a number 1 and 2 (I always take a Senokot Max strength tablet on a Friday to make sure I 'go'). I weigh in the bathroom and unforunately I cannot make sure the scales are in the exact same place on the floor each week because our bathroom is just so small and stupidly laid out that there is nowhere I can leave the scales all the time, so I bring them down from upstairs and put in the bathroom when I need them.
Maybe I worry too much, but I know for a fact that if I was to get on my scales then move them a few feet away and get straight on they would say something different!