I am not too worried about my weight on a week to week basis. I shall look more towards a monthly figure. I was three pounds heavier after my ride on Sunday but that had all but gone today.
That's partly why the "planner" for recording the weights each week is arranged so (it was jwo's original layout I think). It makes it easier to compare your weight with your weight a month previous.
It's why it's important to weigh yourself on the same day of the week or under the same circumstances each week. One would get wildly different weights when measured very dehydrated after a long run without enough water, and 2 hours after that when they've stuffed themselves with food and drank 5 pints of water to try and rehydrate.
I play 5-a-side on a Tuesday night and then go to the pub and have 2 pints. Never more and we rarely miss a week. I've always eaten before I go play 5-a-side and I don't eat afterwards. This makes Wednesday morning a perfect time to weigh myself. Get up, go to the toilet, wash hands and get on the scales.
As you say, just watch the general trend and adjust food/exercise/etc appropriately.