Every day here. I keep track of my cycle, and know how it affects my weight, so the up and down from hormones doesn't bother me.
I actually use the Hacker's Diet tracker to keep track of my weight. The trend line goes down fairly steadily, even if the scale doesn't.
For me, weekly weigh-ins wouldn't protect me from random fluctuations. If I report gain/loss weekly for a challenge, I usually have a gain or maintain every other week. Since I weigh every day, I know if that gain/maintain is a one-off thing, or a regular occurence.