Early morning would seem wise to me, too. The one crucial factor is that you always take measurements and/or weigh at the same time of day.
Not wanting to depend on just my guess, I did try to find a reliable site that answered the question, but nada, nyet, non.
You don't have to do it in the a.m. But if you want to play a little game/experiment, you could try measuring yourself in the morning and then again at some other point of the day regularly (daily, weekly, monthly) just to see how different IF different your results are over time. Would be interesting, huh?