I'm also an obsessive weigher.
For me it's fun and I like to pick out trends (yep, I'm a scientist in real life... go figure!).
Anyways, here's what I've noticed. During my period I either mantain or gain weight with a night loss at the end of it.
Also it looks like when I ovulate I gain a bit for a day or two. If I have a big meal the night before (especially with something heavy like beef) of if I eat dinner later at night then I can expect to see a higher number the next day but a day afterwards it drops back (ditto if I ate something salty).
I'm still nursing too, which mean the actual best time for me is first thing in the morning but AFTER DD has nursed. However, she doesn't always nurse so sometimes I get higher weights because of that.
Oh, and if I weigh myself after exercising than I can be 1-2lbs lighter just due to losing water weight.
It's all fun and I don't let it stress me out. I normally don't update my ticker unless I see the same weight for a couple of days in a row just to avoid fluctuations (also I don't change it when I have my period or am ovulating).
I also pretty much never weigh myself at night because I drink A LOT of water and that number is normally really scary!!!