I think no matter when we choose to weigh ourselves, consistency is key, so I try to at least weigh myself under the same conditions every time. Lately I have been actually counting the post-workout weightloss only, but if that's the only weight I'm checking, then at least each day's weight recorded is consistent when compared to the previous day.
I do check at other times of the day, but lately my fluctuations haven't been that dramatic, so I think I'm getting close to a stable zone. I don't think I'd weigh smack in the middle of the day, though. Usually if I check multiple times, I'll check once in the morning, once after a workout after dinner, and maybe again right before bed, but I only
count the post-workout one.