I would suggest picking one (before or after) and then stick to that. Personally, my hair holds quite a bit of water post-shower, so I usually do my weighing post-pee, pre-shower. I think the body fat scales use water in our bodies to measure fat percentages- something about the little pulses that travel through our bodies. As far as I know, my digi scale is just like my analog scale- step up and hold your breath. I wouldn't worry too much about being adequately hydrated.

There's my $.02 for the day!

HTH-
Nikki