I weigh in periodically, no particular pattern or day/time. Usually three-four times per week, and most often when I'm changing clothes (so either early-morning, mid-afternoon, or late-night). Almost always with just underwear/socks, however.
Weighed myself yesterday around noon when I got home (had no breakfast in a.m., which is unusual), wearing light sweats and a t-shirt. Took a nap, woke up 3 hours later, took a pee, and weighed in 1.5 lb heavier than before the nap/pee. Wtf....
That's why longer intervals between weigh-ins are better. So long as the trend doesn't start going UP, I'm happy.