For the past year, my weight has fluctuated on a daily basis within the same 5 pounds. So I just consider that my weight is somewhere in between that five pounds.
I have a scale that measures body fat. If I am dehydrated, my weight will be lower, but my scale will say that my body fat is much higher - up to 10% higher than if I am fully hydrated. My scale actually loves it when my weight is up but I am fully hydrated - it says that my metabolic age is lower, that my muscle mass is higher, that my body fat percentage is much lower, that my bone mass is higher, that I have less visceral fat. After a long run without enough hydration, my scale freaks out and gives the exact opposite readings. I think it's actually pretty funny. I only consider readings to be somewhat accurate in comparison to other scale readings taken under the same conditions - same time of day, same hydration level, etc. Only the weight itself is to be relied on with this type of measurement - the rest is way too dependant on my hydration level.